The Role of the

Genus Ceanothus in

Western Forest

Ecosystems

This file was created by scanning the printed publication. Mis-scans identified 

by the software have been corrected; however, some errors may remain. 

United States 

Department of 

Agriculture 

Forest Service 

Pacific Northwest 

Forest andRange 

Experiment Station 

General Technical 

Report 

PNW- 182

July 1985
Authors SUSAN G. CONARD is an ecologist at the Forest Fire Laboratory, Pacific 

Southwest Forest and Range Experiment Station, 4955 Canyon Crest Drive, 

Riverside, CA 92507. ANNABELLE E. JARAMILLO is a botanist at the Forestry 

Sciences Laboratory, Pacific Northwest Forest and Range Experiment Station, 

3200 Jefferson Way, Corvallis, OR 97331. KERMIT CROMACK, JR., is an assistant professor, Forest Science Department, Oregon State University, Corvallis, OR 

97331. SHARON ROSE is a soil scientist, 517 Brown Avenue, Fort Collins, CO 

80525. 

This publication reports research involving pesticides. It does not contain 

recommendations for their use, nor does it imply that the uses discussed have 

been registered. All uses of pesticides must be registered by appropriate State or 

Federal agencies before they can be recommended. 

Cover art courtesy of Paula Reed, Oregon State University, Corvallis. 
The Role of the Genus 

Ceanothus in Western Forest 

Ecosystems 

Susan G. Conard, 

Annabelle E. Jaramillo, 

Kermit Cromack, Jr., and 

Sharon Rose 

Compilers 

Report of a workshop held November 22=24, 1982 

Oregon State University, Corvallis, Oregon 

Published by: 

U.S. Department of Agriculture 

Forest Service 

Pacific Northwest Forest and Range Experiment Station 

Portland, Oregon 

General Technical Report PNW-182 

July 1985 
Contents 1 

2 

16 

24 

31 

38 

44 

47 

48 

49 

66 

68 

1. Introduction 

2. The Importance of Ceanothus Species in U.S. Forest Ecosystems 

3. Autecology and Synecology of Ceanothus 

4. Interactions Between Ceanothus Species and the Soil System 

5. Physiological Ecology of Ceanothus and Associated Conifer Species 

6. Effects of Ceanothus on Forest Productivity and Conifer Growth 

7. Research Priorities 

8. Concluding Remarks 

Acknowledgments 

References 

Appendix 1: Scientific Names of Species Cited 

Appendix 2: Workshop Participants 
Abstract Conard, Susan G.; Jaramillo, Annabelle E.; Cromack, Kermit, Jr.; Rose, 

Sharon, comps. The role of the genus Ceanofhus in western forest ecosystems. 

Gen. Tech. Rep. PNW-182. Portland, OR: U.S. Department of Agriculture, Forest 

Service, Pacific Northwest Forest and Range Experiment Station; 1985. 72 p. 

This report was developed from discussions on the role of Ceanothus in western 

forests that took place at a workshop held November 22-24, 1982, at Oregon State 

University, Corvallis, Oregon. The workshop provided a forum for discussing 

research relevant to Ceanothus management. Major topics were autecology and 

synecology; interactions between Ceanothus species and the soil system; the 

physiological ecology of Ceanothus and associated conifers; and the effects of 

Ceanofhus on forest productivity and growth. The workshop was attended by 

scientists and land managers from government agencies, universities, and forest 

industry. This report summarizes available information concerning the biology and 

ecology of Ceanothus species in western forest ecosystems; describes and 

critiques methods, results, and conclusions of past research; discusses current 

investigations; identifies research needs; and includes an extensive bibliography 

and a listing of workshop participants involved or interested in Ceanothus research 

and management. 

Keywords: Ceanothus, competition (plant), ecology (plant), autecology (plant), 

synecology (plant). 
1. Introduction 

The impacts of native, symbiotic, nitrogen-fixing plants on conifer productivity, 

vegetation dynamics, nutrient cycling, and wildlife habitat in forest ecosystems are 

of considerable ecological and economic interest. The two major genera of 

nitrogen-fixing woody plants on forest sites in the western United States are 

Ceanothus and A/nus.-l/ Although Alnus is an important component in coastal and 

mesic low- and high-elevation forests of the Pacific Northwest, the primary nitrogenfixing shrubs on drier and middle-elevation forest sites are species of Ceanothus. 

Several of these species are frequently dominant components in early successional 

vegetation following disturbance such as fire. 

Two recent publications (Gordon and Wheeler 1983, Gordon and others 1979) provide an excellent overview of current research on the effects of nitrogen-fixing 

species on forest site productivity; the biology, ecology, and physiology of the hostsymbiont relationship; options for integrating nitrogen fixers into forest management; and economic considerations in the management of symbiotic nitrogen fixers. Although these publications summarize much of the available data on Alnus 

and other species, the information they present pertaining to Ceanothus is limited. 

A fairly large body of research information exists on the biology and ecology of 

forest species of Ceanothus. The literature indicates that Ceanothus affects many 

components of the forest ecosystem, including nitrogen availability, nutrient cycling, 

conifer establishment and growth, and forage quality for browsing animals. 

The workshop on Ceanothus was initiated because widely divergent opinions exist 

among scientists and land managers concerning the relative benefits and negative 

impacts of Ceanothus. The workshop was structured to provide a forum for discussing major research areas relevant to Ceanothus management. The three major objectives of the workshop were: (1) to summarize available information concerning 

the biology and ecology of Ceanothus species in western forest ecosystems; (2) to 

discuss research methods, results, conclusions, and interpretations; and (3) to identify future research needs. Participants included scientists and land managers from 

government agencies, universities, and forest industry. 

dscientific and common names of species are listed in 

appendix 1. 

1 
Following an overview presentation on the importance of Ceanothus species in U.S. 

forest ecosystems, the workshop was organized into small working groups, each of 

which focused on one of the four topic areas: 

1. The autecology and synecology of Ceanothus. 

2. Interactions between Ceanothus species and the soil system. 

3. The physiological ecology of Ceanothus and associated conifers. 

4. Effects of Ceanothus on forest productivity and conifer growth. 

Past and current research was discussed in each group; the objective was to summarize available information and identify promising techniques for new or improved 

studies. Emphasis was placed on including existing studies on Ceanothus within a 

framework of long-term research objectives. 

Each participant was provided a preliminary bibliography and a list of recommended reading prior to the workshop. Participants were also encouraged to discuss unpublished research. 

This report summarizes and updates workshop presentations and discussions. The 

introductory remarks by Jerry Franklin and the concluding remarks by John Gordon 

are based on transcripts from tapes. Summaries of group discussions (sections 3-7) 

include pertinent published and unpublished results identified by participants, comments on research methods and approaches, and recommendations for future 

research. These summaries were prepared by the compilers. 

2. The Importance of Ceanothus Species in U.S. Forest 

Ecosystem@ 

This paper discusses autecology, distribution, and taxonomic relationships of the 

major forest species of Ceanothus and considers some of the various roles that 

Ceanothus plays in western forest ecosystems. 

3 This section prepared by Jerry Franklin, chief plant ecologist, 

Forestry Sciences Laboratory, Pacific Northwest Forest and 

Range Experiment Station, Corvallis, OR 97331; Charlie Halpern, 

research assistant, Botany Department, Oregon State University, 

Corvallis, OR 97331; Brad Smith, research assistant, Botany 

Department, Oregon State University; and Ted Thomas, research 

assistant, Forest Science Department, Oregon State University. 

2 
Ceanothus is a North American genus of Sierra Madrean origin. Of the 55 species 

in the genus, 51 of them are found in western North America, four are found in the 

Eastern United States, and two are isolated in Florida. There are 44 species in the 

Pacific coast region; 38 of these are restricted to the California floristic province, 

which includes California, Baja California, southwestern Oregon, and western 

Nevada. Six species are found in the Rocky Mountains.’ 

Species in the genus Ceanothus range from mat-forming shrubs less than 0.3 

meter (1 ft) high to small trees as tall as 4 to 9 meters (15-30 ft). Typically, they are 

shrubs 0.9 to 2.4 meters (3-8 ft) tall. The genus contains both deciduous and 

evergreen species. 

Genus sections.-Two sections of the genus are recognized, Euceanothus and 

Cerastes; within each section, the species are highly interfertile. There are many 

known hybrids, which may account for the variability in the taxonomic treatment of 

the genus-our treatment follows McMinn (1942). The potential for hybridization 

and formation of species that can adapt to new environments may be one reason 

the genus has been successful. 

The more ancient section is Euceanothus, and it contains about 60 percent of the 

species (approximately 33). Characteristics of several distinctive species are shown 

in figure 1. Most have retained the ability to sprout. This is also the section in 

SECTION EUCEANOTHUS 

SPINY 

EVERGREEN 

CEANOTHUS FENDLERI 

C. CORDULATUS 

C. INCANUS 

C. VELUTINUS 

C. THYRSIFLORUS 

C. PAR VIFOLIUS 

C. INTEGERRIMUS 

C. SANGUINEUS 

EASTERN 

C. AMERICANUS 

C. OVATUS 

DECIDUOUS I 

Figure 1 .-Ecological characteristics and common forest species 

of section Euceanofhus in the genus Ceanothus. 

3 
which most of the important forest species are found. Both evergreen and 

deciduous species and both spiny and nonspiny species are included. The four 

species in the Eastern United States are found in this section. 

The other section, Cerastes, has about 26 species, and some people believe it 

contains the most long-lived ones. The species in this section do not sprout, and 

only a few of them are important forest species (C, cuneafus, C. prosfratus, and C. 

pumilus). C. greggii is, however, an important desert and chaparral species (USDA 

Forest Service 1937). The purported longevity of these species supports a 

hypothesis that seed reproduction rather than sprouting tends to be characteristic 

of long-lived species. 

Distribution.-Many Ceanofhus species occur in chaparral and desert habitats at 

elevations below forest sites or at the forest-nonforest interface. Approximately a 

third of Ceanofhus species (20 to 25) are forest oriented. These species, with a few 

exceptions, are important components of the communities in most western forest 

zones except in the coldest zones and in moist habitats, such as the Sitka spruce 

zone and the wetter parts of the western hemlock zone. Generally, Ceanofhus 

species are not found in subalpine forests except at the lowest elevations of the 

zone. Ceanofhus distribution is skewed toward the driest forest zones-ponderosa 

pine and the Douglas-fir series-and toward early successional stages of midslope 

zones in the Rocky Mountains, the Sierra Nevada, and the Cascade Range. This 

includes the mixed conifer zone of the Rocky Mountains, the mixed conifer forests 

of the Sierra Nevada, and the midslope forests on the east slope of the Cascade 

Range. 

The West is divided into several major habitat regions (fig. 2). In the northwestern 

area, Ceanothus are very common seral species in some parts of the western 

hemlock zone, in the lowest parts of the Pacific silver fir zone, and especially in 

habitats where Douglas-fir is the climax species. In southwestern Oregon and northern California, Ceanofhus species are represented in all the forest zones, from 

the driest (Douglas-fir) through the more moist tanoak, western hemlock, white fir, 

and red fir zones and are associated with Jeffrey pine on serpentine habitats. In 

the Sierra Nevada and the Coast Range of California, Ceanofhus species are very 

important in the chaparral and extend into the Digger pine and oak woodlands and 

all of the forested zones, including the red fir and Jeffrey pine forests at higher 

elevations. It is also an important genus in the knobcone pine forests. In the northern Rocky Mountains, Ceanofhus is a component of most of the important forest 

zones except for the upper part of the subalpine forest. Ceanofhus are seral 

species of western hemlock and western redcedar zones in northern Idaho and 

western Montana. Utah, Colorado, Arizona, and New Mexico have only one 

species, C. fendleri, which is found primarily in the ponderosa pine zone. A possible hypothesis for the absence of many Ceanofhus species in the southwest is that 

they are poorly adapted to summer precipitation. 

Physiology.-Ceanofhus species are generally intolerant to very intolerant of 

shade. This has important implications for the successional role that they play. The 

genus is also very tolerant of stress and can withstand both cold and hot 

temperature extremes, drought, and poor site conditions. 

4 
Figure 2.-Zones or community series in which one or more 

Ceanothus species play important roles. 

Ceanothus species have the potential to fix nitrogen. Although the occurrence of 

nodules on Ceanothus has been known for a long time, demonstration of the ability 

of the plants to actually fix nitrogen is a relatively recent discovery (Bond 1957). 

The ability of Ceanothus to fix nitrogen has been demonstrated in the laboratory for 

all species that have been examined in detail. Delwiche and others (1965), for example, worked with 12 species using excised nodules and found that in all cases 

when these species are nodulated they are capable of fixing nitrogen. Field 

demonstrations of net nitrogen accretion are much more limited and are somewhat 

controversial. Probably all the species have the potential to fix nitrogen on at least 

some sites at least part of the time. 

5 
Reproduction.-All or most of the species reproduce from seed, which is persistent and is subject to seedcoat dormancy. In the laboratory, most species require a 

hot water treatment and stratification for germination (Reed 1974). In nature, heat 

from fires or from other sources, such as the sun, is believed to be the major 

stimulant to germination. Fire is not absolutely essential for germination of 

Ceanothus seed, but it is the most common and effective way to stimulate germination. Gratkowski’s (1962) work on C. velutinus shows that heating the seed results 

in a permanent irreversible opening of the hilar fissure which allows moisture to 

penetrate. Heat does not affect the seedcoat itself. 

Sprouting is not the primary means of reproduction in the genus, although most of 

the forest species exhibit at least some ability to sprout. There is some confusion 

in the literature regarding the distinction between sprouting and nonsprouting 

species. This confusion may result from the fact that species without a root crown 

burl cannot tolerate repeated, intense burning. This could be why many ecologists 

say that a particular Ceanothus species does not normally sprout. This is especially puzzling as many Ceanothus species occur in environments such as chaparral 

where fire is common. In fact, most or all species have some sprouting ability, 

although the species in the section Cerastes do not normally do so. Even though 

disturbance is necessary to stimulate seed germination, fire can be either too frequent or too intense, and can eliminate Ceanothus species. Seed and plants can 

be destroyed with repeated burning. For example, if Ceanothus is windrowed, the 

seed can be piled up within intense fuel concentrations. In such situations, much 

seed is destroyed, and establishment of Ceanothus species is often very low. 

Wildlife managers express concern about reduced Ceanothus occurrence caused 

by fire control programs. Although some foresters may find it hard to imagine insufficient Ceanothus, wildlife experts in some national parks in the Sierra Nevada are 

concerned that fire control programs may destroy a favorite browse for wildlife. 

Occurrence.--The successional roles of Ceanothus species are a consequence of 

their combined intolerance of shade and a need for disturbance for regeneration. 

Ceanothus generally occurs in one of two situations: First, it can be a regular component of forest communities that lack a closed canopy and that have some level 

of disturbance to allow for reproduction of the species. For example, in chaparral, 

some woodlands, Jeffrey pine stands, and even in some ponderosa pine and 

Douglas-fir stands, the canopy never gets sufficiently dense to shade out the 

species. Ceanothus could be a basic component of the stable forest on such sites. 

Second, Ceanothus may exist as an early successional species on sites that 

develop a closed forest canopy; for example, snowbrush (C. velutinus) in the mixed 

conifer forests of the northern Rocky Mountains or in the western hemlock zone of 

the Cascade Range. These are situations in which Ceanothus reproduces following 

a disturbance such as logging, burning, or wildfire. It grows until it is overtopped 

by the trees, and it persists as seed until the next major disturbance. 

In many young forests, the canopy can be dense enough to shade out Ceanothus, 

but in mature stands there are many canopy openings. Many of the Sierra mixed 

conifer forests, for example, have a very open canopy and in a mature forest, 

Ceanothus can be a permanent member of those forested communities. Some of 

the Douglas-fir climax sites in southwestern Oregon and some of the drier sites in 

the Rocky Mountains are similar. Whether the genus is pioneer or climax, many 

Ceanothus species also have the ability to retard succession by forming 

br us hf ields. 

6 
I 

I--- 

I 

Figure 3.-Distribution of Ceanorhus cunearus. Range information 

from McMinn (1942), Hitchcock and Cronquist (1961), and Sampson and Jesperson (1963). 

Ecology and distribution of selected western Ceanothus species-Ceanothus 

cuneatus, (wedgeleaf ceanothus or buckbrush, fig. 3) is primarily a chaparral 

species, but it also occurs in ponderosa pine and other dry forests. Typical of 

shallow soils and dry sites, it is a very drought-resistant species. It is a vigorous, 

aggressive species that can form impenetrable thickets. Because of its palatability, 

it is an important winter food for deer in many areas. 

7 
Ceanothus fendleri (Fendler ceanothus, fig. 4) occurs in the ponderosa pine zone in 

the Rocky Mountains. It also occurs in some of the drier Douglas-fir habitats of the 

Southwest (Alexander and others 1984) but is generally absent from white fir 

forests. A nonsprouter, it germinates following fire and is a component of the stable 

mature forest. C. fendleri is an important browse plant for deer and is often mentioned in connection with deer management. 

a 
c" - 

NEVADA I 

I 

I 

I 

I 

I 

I 

I 

I 

I 

I---- 

I /' 

I COLORADO 

I 

1 

I 

I 

I 

I 

--- I---- 

I 

I 

I 

UTAH I 

ARIZONA I NEW MEXICO 

c- c 

Figure 5.-Distribution of Ceanothus sanguineus. Information 

from McMinn (1942), Hitchcock and Cronquist (1961), and personal observations of the authors. 

Ceanothus sanguineus (redstem ceanothus, fig. 5) is found primarily in the 

ponderosa pine zone and in parts of the mixed conifer and western hemlock zones 

in both the Cascade Range and in the northern Rocky Mountains. In the 

Cascades, it is common on sites in the Douglas-fir habitat series, particularly after 

logging and burning. C. sanguineus is considered excellent browse for wildlife. 

9 
Figure 6.-Distribution of Ceanofhus fhyrsiflorus and C. incanus. 

Information from McMinn (1942), Munz and Keck (1973), and 

Sampson and Jesperson (1963). 

Ceanothus thyrsiflorus (blueblossom, fig. 6) occurs in southern Oregon and northern California coastal regions on both forest and chaparral sites. Ceanothus incanus (fig. 6) is often associated with coast redwood. It occurs in coast redwood 

and mixed evergreen forests along the northern California coast. 

10 
MONTANA I I 

I 

I 

1 

I--- 

------- - --- 

Figure 7.-Dist r i bu t ion of Ceanothus integerrimus. Information 

from McMinn (1942), Hitchcock and Cronquist (1961), Sampson 

and Jesperson (1963), and personal observations of the authors. 

Ceanothus integerrimus. (deerbrush, fig. 7) is widely distributed both geographically 

and ecologically and occurs from the dry ponderosa pine forests into the western 

hemlock and white fir zones. It is also associated with Coulter pine at lower elevations in southern California. Gratkowski (1961a) identified C. integerrimus as a major 

brush species in the Umpqua, Rogue, and Cascade regions of southern Oregon. It 

is also found in chaparral of the southern Rocky Mountains. Will Moira notes that 

this species can occur in chaparral and in riparian forests in Arizona and 

southwestern New Mexico. Cronemiller (1959) indicates that it can form dense 

stands, it can sprout, and it can be eliminated with repeated disturbance. According to the Range Plant Handbook (USDA Forest Service 1937), it is “one of the 

most valuable western browse plants.” 

9 Personal communication, Will Moir, USDA Forest Service, 

Southwestern Region, Albuquerque, NM 87102. 

11 
MONTANA I 

I 

1 C. PARVIFOLIUS 

I I I 

-5 AT - 

I 

- I IDAHO 

ca 

I 

1 

1- -- II 

I 

I 

I 

.. 

I ARIZONA I NEW MEXICO I \ 

I 

I 

I 

\ 

\ 

/ 

\ I 

I 

CALIFORNIA a 1 I 

/ 

0 

I 11 C. CORDULATUS 

Figure 8.-Distribution of Ceanothus cordulatus and C. pawifohus. 

Information from McMinn (1942), and Sampson and Jesperson 

(1963). 

Two California species are Ceanothus cordulatus (mountain whitethorn) and 

Ceanothus pawifohus (fig. 8). C. cordulatus is widespread geographically; it occurs 

from southern Oregon to Baja California and ranges from ponderosa pine forests 

into the red fir zones. Commonly associated with Jeffrey pine and ponderosa pine, 

this species increases with disturbance and forms very dense and difficult-topenetrate brushfields. It also is persistent in the open forests on the midslopes of 

the Sierra Nevada and transverse ranges in California. C. parvifolius Is similar but it 

lacks thorns and has a more limited range. 

12 
t 

I--- 

----------- WYOMING I 

c. PROSTRATUS 

Figure 9.-Distribution of Ceanothus prostratus and C. pumilus. 

Two that form mats are Ceanothus prostratus (squaw carpet) and Ceanothus pumilis 

(fig. 9). C. prostratus has a wide elevational range and occurs on dry, lithosolic 

sites. It is common in Jeffrey pine and ponderosa pine forests in the southern 

Cascade Range and in the Sierra Nevada. C. pumilis is primarily a serpentine 

species found in association with Jeffrey pine. 

13 
I 

I 

I 

I 

I 

I 

I 

I 

I1 

Figure 10.-Distribution of Ceanothus velutinus. Information from 

McMinn (1942), Mueggler and Campbell (1982), and Youngblood 

and Mueggler (1981). 

Ceanothus velutinus (fig. lo), one of the most important forest species, has two 

widespread varieties--snowbrush (var. velutinus) and varnishleaf (var. laevigatus). C. 

velutinus is widespread geographically and grows in a wide variety of habitats. In 

California, it occurs in mixed evergreen, red fir, white fir, mixed conifer, Jeffrey pine, 

and ponderosa pine forests. In Oregon, it is found in mixed conifer, mixed 

evergreen, western hemlock, and lower Pacific silver fir zones. In the interior Rocky 

Mountain region, it is found in ponderosa pine, Douglas-fir, mixed conifer, and 

lower subalpine zones. C. velutinus is important in brushfield formation. On sites 

where it develops best it is seral-a successional species that reproduces through 

persistent seed that is perhaps three or four centuries old. It is eliminated as a 

14 
Conclusions 

shrub when the canopy closes. It can persist in mature forests on some sites, 

however, such as the decomposed granitic habitats in eastern Montana, which 

belong to the Douglas-fir series. 

Ceanothus velutinus is the most extensively studied of the forest Ceanothus 

species. The studies include investigations of the ecological role, autecology, role 

in nitrogen cycling, and competitive abilities of the species. Wahlenberg (1930) investigated the vigor and mortality of ponderosa pine seedlings planted in, at the 

edge of, or outside C. velutinus clumps or bushes. Fire stimulates germination and 

sprouting but is not absolutely required for C. velutinus to reproduce. Halpern’s 

data (unpublished) from the H. J. Andrews Experimental Forest, Willamette National 

Forest, show that C. velutinus may occur on sites that are disturbed but not 

burned. It develops more rapidly and more completely on sites that are burned, but 

occasional seeds can germinate on sites that are not burned. C. sanguineus is 

even less dependent on fire than is C. velutinus and it can develop into good 

stands on unburned sites. 

Ceanothus species are tolerant of stress but intolerant of shade, are capable of 

nitrogen fixation, can reproduce from persistent seeds that require heat to germinate, have a limited ability or no ability to sprout, and require moderate disturbance to reproduce. 

Ceanothus is a very important genus as a food source for wildlife. The Range Plant 

Handbook (USDA Forest Service 1937) describes C. sanguineus, C. thyrsiflorus, and 

C. fendleri as being some of the most important browse species in their respective 

ranges. 

Ceanothus is important it?-site amelioration. The genus does have nitrogen-fixing 

and other soil-building attributes. It is tolerant of severe sites, and it can play a soilbuilding role on these sites. 

Ceanothus species, especially those such as C. prostratus that form mats, are important in erosion prevention. Several species have root systems that can help to 

prevent mass soil movements. 

Ceanothus species (in particular C. prostratus and C. integerrimus) have been mentioned as nurse crops. In dense Ceanothus thickets, however, some species (C. 

velutinus, C. integerrimus, C. cordulatus, and C. cuneatus) are regarded as very 

strong competitors that can retard the establishment and growth of conifers. 

The roles and the controversies surrounding Ceanothus species provide several 

researchable problems. Quantitative data are needed on the actual roles that 

Ceanothus plays on various sites. There is a need for better predictive information 

on Ceanothus. For example, where will Ceanothus occur? What role does 

Ceanpthus play in nitrogen fixation on a specific site? How serious are Ceanothus 

species as competitors? How important is Ceanothus competition over the rotation 

of a conifer forest? Do the positive contributions made by Ceanothus to a forest 

system offset its competitive impact on conifers? 

15 
3. Autecology and Synecology of Ceanothus 

Autecology 

Seed Production, 

Germination, 

and Survival 

Seed production.-Ceanothus species that have been studied appear to be prolific 

seed-producers. Although no published data are available on the age when seed 

production begins, field observations indicate that C. integerrimus begins to produce seed at about 4 years of age and C. velutinus begins seed production after 6 

to 10 years in northern California.3 Even less information is available for sprouts. 

Sprouts of C. velutinus can produce prolific seed crops 8 years following cutting.3 In general, sprouts of most species begin to produce at least some seed 

after 3 to 6 years. 

More is known about numbers of seeds produced. Estimates for various Ceanothus 

species of numbers of seeds stored in the duff range from 4.6 x 106 viable seeds 

per hectare (1.9 x 10Vacre) for C. cordulatus and C, parvifolius combined in California (Quick 1956) to 2.5 x 105 seeds per hectare (1 x 10Vacre) for C. velutinus in 

Oregon (Gratkowski 1962). Three hundred seventy-five thousand germinated seedlings per hectare (150,00O/acre) and 150,000 to 600,000 germinated seedlings per 

hectare (60,000 to 240,000lacre) have been reported for C. integerrimus in California 

(Quick 1956) and C. sanguineus in Idaho (Hickey and Leege 1970), respectively. 

The generally large seed reservoir in the soil where Ceanothus plants are abundant 

apparently represents only a small percentage of the seed produced annually. Based 

on 5 years of sampling, seed production by C. greggii and C. leucodermis on 

chaparral sites in southern California varied greatly from year to year, with production ranging from 47,000 to 29.2 x 106 seeds per hectare (18,800 to 11.6 x 106/acre) 

for C. leucodermis and from 0 to 170 x 106 seeds per hectare (68 x 10Vacre) for C. 

greggii (Keeley 1977b). Seed production of mature Ceanothus plants may vary 

greatly from year to year as a function of the carbon stored by plants in the 

previous year’s and the current year’s precipitation. In chaparral species, a significant percentage (90 to 99 percent) of the annual seed production is eaten by 

rodents, with a small percentage consumed by birds and harvester ants.> 

3 Participants in this discussion group were: Tom Beebe, Ray 

Boyd, Paul Dunn, Dick Fredricksen, Charles Halpern, Ed Harshman, Miles Hemstrom, Howard Horowitz, Annabelle Jaramillo, 

Bruce Kelpsas, Paul Lauterbach, Phil McDonald, Don Minore, 

Doug Robin, Bob Sanders, Bill Stein, and Howard Weatherly. 

Discussion Leaders were Tom Atzet and John Tappeiner. 

3 Personal communication, P. McDonald, Pacific Southwest 

Forest and Range Experiment Station, 2400 Washington, 

Avenue, Redding, CA 96001. 

3 Personal communication, S. Conard. 

9 Personal communication, R. Quinn, California Polytechnic 

University, San Luis Obispo, CA 93407. 

16 
Conclusions drawn from the discussion of seed production by Ceanothus were: 

1. In years of high seed production, Ceanothus species are capable of producing 

more seeds in a single year than are stored in.the soil. 

2. The number of seeds in the soil can be expected, to vary from year to year as a 

function of both seeds produced and seeds utilized by consumers. 

3. Substantial seed reservoirs exist in the soil in areas that have been dominated 

by Ceanothus species. 

Seed longevity.-Seeds of Ceanothus species are thought to be long-lived. 

Gratkowski (1962) reports viable C. velutinus seeds in the duff of conifer stands 

over 200 years old. The only direct information on longevity, however, comes from 

work by Quick and Quick (1961) who obtained excellent germination in Ceanothus 

seeds known to be 9 to 24 years old. More studies in this area are needed. 

Seed dissemination.-Because the seeds of Ceanothus species are generally 

small (2-4 millimeters (0.08-0.16 in) long), smooth, hard, and round, they tend to 

stay where they fall, gradually filtering into the duff and soil, unless they are carried off by small animals (rodents, birds, ants). 

When Ceanothus seeds ripen, they are forcibly ejected from the pods. One chaparral species, C. crassifolius, has been observed to shoot seeds an average of 3 

meters (10 ft)3 

Seed germination.-Laboratory studies of treatments to stimulate germination of 

Ceanothus species were summarized by Reed (1974). The ability of the seeds to remain viable for years under field conditions is a function of a strong seedcoat dormancy. As the seeds dehydrate, their hilar fissures act as one-way valves and prevent moisture from being imbibed. In nature, dormancy is broken when the hilum is 

fractured (Gratkowski 1962). Fire is the most common agent stimulating germination, although high soil temperatures caused by solar radiation and mechanical 

abrasion may also be factors. Laboratory studies have shown that either soaking in 

hot water at 80 to 90 OC (176 to 194 OF) or dipping in boiling water for as little as 

10 to 30 seconds to as much as 5 minutes will stimulate germination of many 

Ceanothus species. Various combinations of immersion of seeds in sulfuric acid 

and soaking them in gibberellin or in thiourea will also stimulate germination. Cold, 

moist stratification normally increases germination significantly following hot water 

or other heat treatments (Gratkowski 1973). At least one chaparral species (C. 

crassifolius) averages about 15 percent germination if it is placed in water at 70 OC 

(160 OF) and left in the same water at room temperature, suggesting that cold 

stratification is not required for all species. Even in this species, though, germination is enhanced considerably by cold stratification (M. Poth, unpublished data). In 

dry soils, temperatures of 120 to 140 OC (248 to 284 OF) are normally lethal to 

Ceanothus seeds. 

9 Personal communication, P. Dunn, Forest Fire Laboratory, 

4955 Canyon Crest Drive, Riverside, CA 92507. 

17 
Plant Growth and 

Stand Development 

In field situations, autumn slash burnings have produced four times as many C. 

sanguineus germinants as have spring burns, presumably because of the hotter 

temperatures common in autumn burns and the cold, wet stratification that follows 

with winter weather. Germination following spring burns is often delayed until the 

following spring (Leege 1968). On two experimental watersheds in the H. J. Andrews Experimental Forest, Willamette National Forest, in western Oregon, the 

highest crown covers of both C. sanguineus and C. velutinus occur in the most 

heavily burned areas (C. Halpern, unpublished data). Chaparral studies have 

shown that soil temperatures in hot or moderate fires may reach 150 OC (302 OF) 

-a lethal temperature-down to 2.5 centimeters (1 in) in dry soils (DeBano and 

others 19i7). 

Seedling survival.-Although large numbers of seeds of Ceanothus species typically germinate after fire, mortality is high, especially in the first few years following 

germination. For example, in the Sierra Nevada, C. integerrimus decreased from 

750,000 seedlings per hectare (300,00O/acre) in the first year to 25,000 per hectare 

(10,0001acre) after 10 years; 6,250 (2,50O/acre) after 20 years; and less than 1,250 

per hectare (500/acre) after 30 years (Cronemiller 1959). Rapid early mortality has 

also been observed from C. velutinus, C. sanguineus, C. cuneatus, and C. megacarpus on some sites. In pure stands, however, densities of these species appear to 

level off after 5 or 6 years. Density of 5-year-old C. velutinus in the mixed species 

shrub community near Mount Shasta, California, ranges from 2,250 to 3,750 per 

hectare (900 to 1,5001acre) (P. McDonald, unpublished data). Schlesinger and 

others (1982) report decreases in C. megacarpus seedlings on sites from 101,250 

per hectare (40,5001acre) at age 1 to 20,250 per hectare (8,1OO/acre) at age 20. 

Because Ceanothus species are generally intolerant of shading, they begin to die 

out after conifers grow through the shrub canopy (approximately 10 years on some 

C. velutinus sites in Oregon; considerably longer on some sites in the Sierra 

Nevada). Only a small percentage of the viable seeds in the soil germinate after a 

typical fire. Because of large numbers of seeds in the soil, even relatively low germination is sufficient to produce high densities of Ceanothus shrubs. 

Seedlings.-Both varieties of C. velutinus (var. laevigatus and var. velutinus), as 

well as other Ceanothus species, exhibit sigmoidal height growth. Height growth is 

slow until age 4 or 5 years in var. velutinus and until age 3 years in var. laevigatus 

and then is rapid until about age 10 years when it begins to level off. A typical C. 

velutinus var. velutinus stand in Oregon or California reaches a height of 0.6 to 2.5 

meters (2 to 8 ft) after 10 years. Stand height sometimes decreases between age 

15 and 50 as older stems collapse. Maximum stand height depends on site 

characteristics. In the Cascade Range (McKenzie Ranger District, Willamette National Forest), the number of C. velutinus plants decreased from 3,700 at age 4 

years to less than 400 at age 12 years (E. Harshman, unpublished data). The data 

showed a rapid exponential decline in numbers required for full cover until about 

age 8 or 9 years, when the decline became more gradual as shrubs approached 

maximum size. Little site-specific information is available, however. In one study in 

the northern Sierra Nevada, C. integerrimus was reported to grow 7 to 11 centimeters (3 to 4 in) the first year, and 20 (8), 30 (12), and 36 centimeters (14 in) in 

years 2, 3, and 4, respectively. Total height growth after 5 years ranged from 0.8 to 

1.3 meters (2.5 to 4.3 ft). This species begins to spread out rapidly, horizontally and 

vertically, in the third year (I? McDonald, unpublished data). Cronemiller (1959) 

18 
reported similar growth data from the Stanislaus National Forest in the Sierra 

Nevada. As with C. velutinus, growth varies considerably from one site to another. 

Sprouts.-Although many Ceanothus species are vigorous sprouters following top 

removal, few data are available on the growth rates of sprouting shrubs. Sprout 

growth of 51 to 65 centimeters (20 to 25 in) after 1 year and 1.0 to 2.1 meters (3 to 

7 ft) after 5 years have been observed for C. velutinus.* C. integerrimus sprouts 

grew 75 centimeters (30 in) in the first year after burning on one site (Cronemiller 

1959). Sprout growth rates also vary from site to site. 

The ability to sprout may also be a function of the age and vigor of the plant when 

cut. In one study in the Cascade Range, when C. velutinus plants were cut, the 

percentage of sprouting plants decreased from 100 percent for 5-year-old plants to 

50 percent for ll-year-old plants. Almost no sprouting was observed on 13-year-old 

plants (R. Sanders, unpublished data). In the Sierra Nevada, however, vigorous 

sprouting was observed following cutting of 35- to 50-year-old C. velutinus (Conard 

and Radosevich 1982a). Again, these patterns vary from site to site. 

Productivity.-The interest in Ceanothus species as browse for big game has led 

to several studies of annual stem growth and biomass production. Leege and 

Hickey (1975) report average annual twig growth for 32- to 36-year-old C. sanguineus plants in Idaho ranging from 23.6 to 30.5 centimeters (9.3 to 12 in). Annual 

growth was significantly correlated with May to August precipitation totals and was 

higher on south than on east or west aspects. They also observed substantial twig 

dieback, especially on older plants, during periods of severe winter temperature 

fluctuations. Winter twig dieback was also reported by Young and others (1948) for 

C. sanguineus in northern Idaho and by Stickney (1965) on C. velutinus in Montana. 

Winter desiccation of foliage is a common occurrence on C. velutinus in the 

Cascade Range in Oregon and in the Sierra Nevada.2 Little is known about how 

this dieback may affect either annual or long-term productivity. 

Kie (unpublished data) has recently developed models relating annual production of 

twigs and foliage by C. integerrimus and C. cordulatus to measures of shrub 

volume and to overstory crown closure in the southern Sierra Nevada. Published 

models are available that relate C. integerrimus browse production to stem 

diameters (Bartolome and Kosco 1982) and C. cordulatus browse production to 

canopy area of shrubs (Eberlein 1982). 

Biomass and nutrient cycling.-Most available biomass and nutrient cycling data 

are for C. velutinus. This species accumulates biomass rapidly. Martin and others 

(1981) report aboveground biomass values of 36 metric tons per hectare (16.2 

t/acre). Zavitkovski and Newton (1968) and Youngberg and Wollum (1976) report accumulation rates of 4 000-5 000 kilograms per hectare per year (3,560-4,450 

Ib/acre) for the first 10-15 years with slower rates at later stages. C. velutinus 

Personal observation, S. Conard. 

3 Personal observation, S. Conard. 

19 
biomass is high in nitrogen, and stands older than 10 years may contain more than 

500 kilograms per hectare (445 Ib/acre) of nitrogen. The forest floor may contain an 

additional 300 kilograms per hectare (267 Ib/acre) of nitrogen. Nitrogen cycling in 

aboveground litterfall is even more striking. Zavitkovski and Newton (1968) measured an average of 90 kilograms per hectare (80 Ib/acre) per year in C. velutinus 

ecosystems. This rate of nitrogen recycling in litterfall exceeds other temperate 

forest ecosystems except those containing nitrogen-fixing plants such as Alnus. 

C. megacarpus is also a prodigious biomass producer and nitrogen cycler. Schlesinger and others (1982) recorded biomass accumulation rates of 3 000 kilograms 

per hectare (2,670 Ib/acre) per year over 20 years. A 22-year-old stand contained 

480 kilograms per hectare (427 Ib/acre) in biomass, 200 kilograms per hectare (178 

Ib/acre) in the forest floor, and 70 kilograms per hectare (62 Ib/ac) in annual litterfall. The values for this chaparral species are similar to those noted previously for 

montane C. velutinus and much greater than average for nonnitrogen-fixing chaparral species. 

Root distribution.-Root systems of Ceanothus vary considerably from species to 

species. Most species are deep rooted with a large number of fine roots near the 

soil surface. A species with shallow roots is C. greggii, a chaparral species that has 

over 90 percent of its roots in the top 30 to 40 centimeters (12 to 16 in) of soil 

(Kummerow and others 19i7, Miller and Ng 19i7) and a root-to-shoot biomass ratio 

of 0.25 to 0.39 (Miller and Ng 19i7). Roots of C. greggii var. vestitus, on the other 

hand, have been found down to 4 meters (13 ft) in chaparral (Hellmers and others 

1955). C. prostratus roots may extend down to 1.5 meters (5 ft) and may even 

penetrate the C soil horizon (Tappeiner and Helms 1971). 

C. velutinus roots extend to depths of 1.8 to 2.4 meters (6 to 8 ft) and extend 

laterally well past the crown of the plant. In southwestern Oregon, C. cuneatus 

seedlings had roots averaging 20 centimeters (8 in) in length after one growing 

season (Gratkowski 1961b). Maximum length of roots was reported for C. leucodermis in southern California by Hellmers and others (1955). This species had a maximum rooting depth of 6.4 meters (21 ft), and the lateral extent of the roots was approximately 2.3 times that of the aerial parts. 

Longevity.-Compared to other genera of shrubs, Ceanothus are short lived. This 

may be because these species are eventually shaded out by overtopping vegetation; overtopping begins to occur at 10 to 75 years on sites where conifers are present. Individual 40-year-old plants of C. velutinus and C. integerrimus have been 

observed (Cronemiller 1959). In the Sierra Nevada, vigorous C. velutinus were still 

present in a brushfield that had resulted from a fire 50 years earlier (Conard and 

Radosevich 1982a). In the Cascade Range, however, mature C. velutinus are often 

subjected to heavy injury to root crowns from snow (Zavitkovski and Newton 1968), 

which can lead to senescence of stands. The functional life span of C. velutinus on 

many sites in the Cascades is believed to be between 20 and 40 years before 

shading by conifers and snow damage begin to decrease its vigor. Observations in 

the Rocky Mountains have suggested that periodic fire is necessary to maintain the 

vigor of C. sanguineus. Without fire, plants of C. sanguineus may be replaced by 

other shrub species, such as Holodiscus or Prunus. 

20 
Table l--Browse value and protein content of Ceanothus foliage 

Species Deer Cattle Sheep Crude protein 

Percent 

C. cordulatus 1-2 4 3 6-1 5 

C. cuneatus 3 4 2-3 8-1 5 

C. integerrimus 1-2 2-3 1-2 12-20 

C. th yrsi florus 2-4 3-4 2-3 -- C. prostratus 2-3 5 4-5 5-1 0 

C. velutinus 3 5 4 up to 17 

4 Browse value: 5 = useless, 4 = poor, 3 = fair, 2 = good, 1 = excellent. 

Source: Sampson and Jespersen 1963. 

Biotic agents.-Ceanothus foliage, twigs, and seeds are an important food source 

for wildlife and domestic grazers. The foliage of Ceanothus is high in protein and 

can be an attractive spring and summer forage for deer, elk, cattle, and sheep. 

Management to favor production of Ceanothus for browse is common on big game 

ranges (Hickey and Leege 1970, Leege 1979). Palatability and protein content of 

several major Ceanothus species that occur in California are summarized in table 

1. One species that is heavily browsed by elk and deer is C. sanguineus. On five 

sites in Idaho, 25 to 80 percent of the annual growth was removed by elk 

(McCulloch 1955); deer use of this species can reach similar levels (Thilenius and 

Hungerford 1967). C. sanguineus and C. integerrimus are preferred over C. velutinus 

as browse. Snowshoe hare are also heavy browsers of Ceanothus sanguineus on 

some sites (Asherin 1974). Rodents (with an assist from birds and ants) may eat as 

much as 99 percent of the annual seed crop of Ceanothus on chaparral sites.> 

Predation could have profound effects on Ceanothus establishment after disturbance 

if similar seed predation occurs on forest sites. 

A large number of biotic agents, such as birds, insects, mammals, and fungi, have 

been shown to affect Ceanothus species. Birds often act as seed dispersal agents. 

Several insect species heavily infest seeds of Ceanothus species in some years 

and can cause substantial loss of viable seeds. Reductions of 9 to 27 percent in 

seed production were reported for C. sanguineus and 80 percent for C. greggii 

following infestations by wasps, weevils, and caterpillars (Bugbee 1971, Furniss and 

others 1978). The California tortoise shell butterfly can severely defoliate both C. 

velutinus and C. integerrimus.3 E/ Although no quantitative data are available, 

plants seem to recover within a few weeks from severe infestations. 

Personal communication, R. Quinn, California Polytechnic 

Personal communication, A. Shapiro, University of California, 

Davis, CA 95616. 

Personal communication, P. McDonald, Pacific Southwest 

Forest and Range Experiment Station, 2400 Washington Avenue, 

Redding, CA 96001. 

University, San Luis Obispo, CA. 93407 

21 
Synecology 

Fungi may be another important biotic agent that affects Ceanothus species on 

forest sites. Armillaria mellea parasitizes C. velutinus by girdling roots mechanically 

with rhizomorphs (Tarry 1968). Because this fungus is not host specific, infections 

in Ceanothus stands could provide a source for attack of conifers growing on the 

same site. The Cytosporina state of the fungus Eutypa armeniacae has been 

reported to cause stem dieback of several Ceanothus species (Moller and others 

1971a, b). 

Effects of some of these biotic agents might be confused with foliage or twig 

dieback caused by winterhjury, as discussed earlier. Additional information on how 

these biotic agents affect Ceanothus species and their associates would be useful 

to those managing growth and distribution of Ceanothus species in forest systems. 

Ceanothus species occur over a wide geographic area and have a large number of 

associates in many plant communities. Much of the known information on distribution patterns of individual species was presented earlier. Little quantitative information on productivity, community ecology, and successional relationships of 

Ceanothus species in forest ecosystems is available. 

The distribution of sprouting and nonsprouting species of Ceanothus appears to be 

related to frequency of fires. Nonsprouting species are more common than 

sprouting species, and more sprouting species occur in the Sierra Nevada, where 

lightning-caused fires are more frequent, than at low elevations. Keeley (1977a) 

discusses potential evolutionary reasons for these differences. 

Information describing forested plant communities in which the major Ceanothus 

species occur has been reported for Oregon and Washington (Franklin and 

Dyrness 1973), the west side of the Cascade Range in Oregon (Hemstrom and 

others 1982), northeastern Washington (Daubenmire 1953), Montana (Pfister and 

others 1977), and California (Barbour and Major 19T7). 

Seral status of Ceanothus species.-Although few data on specific seral sequences are available (Conard and Radosevich 1982b), Ceanothus species are 

generally considered pioneers because of their fire-dependent reproductive 

strategies. Where they are not shaded out by taller shrubs or trees, however, some 

species can persist as a long-term, semipermanent component of the vegetation. 

Observations suggest that C. cuneatus may form a climax vegetation type on some 

sites in southwestern Oregon and that C. velutinus is part of the climax vegetation 

in open ponderosa pine woodlands on the east side of the Cascade Range. 

22 
Ceanothus species as indicators of site productivity for conifers.-Based largely on 

the field experience of the participants in this session, several relationships between site quality and the species of Ceanothus present are suggested and 

sum m arized : 

Locat ion 

Southwestern 

Oregon 

Idaho 

Eastern 

Was hi ngton 

Sierra Nevada 

(California) 

Northern 

California 

Ceanothus species according to site quality 

Best sites &Worst sites 

(wet) (dry) 

velutinus + integerrimus + prostratus + cuneatus + pumilis 

sanguineus - velutinus 

velutinus +sanguineus 

diversifolius +integerrimus + prostratus + lemmoni + cuneatus 

integerrimus prostratus I_) lemmonii +cuneatus 

Intuition has led to different conclusions from one region to another. This discussion group concluded that good information on correlations among distribution of 

Ceanothus species and site quality, moisture availability, and temperature are sorely lacking. 

Research Needs The following needs were identified for future research on the autecology and 

synecology of Ceanothus species: 

1. Quantifying optimum and extreme values of environmental factors such as site 

quality, elevation, aspect, plant moisture stress, and degree days for growth and 

establishment of each species of Ceanothus. 

2. Quantifying interactions among Ceanothus species and their associates in the 

major (forest) plant communities where they occur. 

3. Defining the autecological characteristics of individual species. Some of the most 

critical are: age at onset of seed production (for seedlings and sprouts), longevity 

of plants, duration of viable seed in the soil, influence of shrub age on sprouting 

capacity, and rates of shrub growth and canopy development. 

23 
4. Interactions Between Ceanothus Species and 

the Soil Systernw 

Ceanothus is one of the most widespread genera of nodulated, nonleguminous, 

nitrogen-fixing plants in North America. Many of the nitrogen-fixing species in this 

genus occur in the western United States and are integral components of forest 

ecosystems (Youngberg and Wollum 1976, Zavitkovski and Newton 1968). Nitrogen 

(N) fixation occurs in nodules that range from simple lobes or clusters a few millimeters across, which are white when fresh, to large, brown lobes 21 centimeters (8 

in) across (Bond 1963, Furman 1959, Gardner 1976). Fixation of atmospheric 

dinitrogen (N2) has been reported in the field and confirmed in the laboratory 

(Delwiche and others 1965, McNabb and Geist 1979, Russell and Evans 1966). 

In 1890, Beal described hyphae and swollen structures inside nodules of 

Ceanothus. More recently, Furman (1959) and Kummerow and others (1978a) detail 

the filamentous nature of the nitrogen-fixing endophyte. Isolation and culture 

techniques for Frankia have been developed (Lalonde 1979), and the endophytes 

have been isolated and cultured for C. americanus by D. Baker and for C. 

crassifolius and C. oliganthus by M. Poth.3 It is known from microscopic observations and from inferences drawn from cultures of the alder endophyte that the 

microorganism is filamentous and encapsulated by a host membrane (Lalonde and 

Knowles 1975). The filaments terminate in swollen tips or vesicles that appear to be 

the site of N2 fixation (Kummerow and others 1978a). 

Although all Ceanothus species tested to date are capable of fixing N2 if a suitable 

inoculum of the endophyte is present (Delwiche and others 1965), little is known 

about host-specificity and environmental influences. In the field, both the degree of 

nodulation and the rate of N2 fixation (or accretion) vary greatly among species and 

among sites (McNabb and Geist 1979, Youngberg and Wollum 1971). Possible 

causes for these observed variations include in herent differences among species of 

host and endophyte, physical and biological site factors, environmental conditions, 

and variability in assessment and measurement methods and research designs. 

11 Participants in this discussion group were: Ray Armstrong, 

Dick Fredricksen, Martin Jurgensen, Logan Norris, Dan Binkley, 

John Marshall, Dave McNabb, Dave Perry, Harold Evans, Bill 

Schlesinger, and Jim Boyle. The discussion leaders were Sharon 

Rose and Kermit Cromack. 

2/ Personal communication, M. Poth, Forest Fire Laboratory, 4955 

Canyon Crest Drive, Riverside, CA 92507. 

24 
Factors Affecting Nodulation and N2 Fixation.--Chemical, physical, and biological 

characteristics of the soil will affect both the infectivity and effectivity of the 

N2-fixing endophyte and will influence the rate of N2 fixation. 

High levels of ammonium nitrogen in the soil decrease N2-fixation rates of C. 

velutinus in laboratory conditions (Rodriguez-Barrueco and others 1970), but this 

has not yet been tested experimentally in the field. The suppression of N2 fixation 

by high soil N levels is widespread in both the legumes and other families of 

N2-fixing host plants (Rodriguez-Barrueco and others 1970). Other macronutrients 

known to influence nodulation and N2 fixation in C. velutinus include calcium, 

phosphorus, and sulfur (Scott 1973). Evidence for the broad applicability of these 

results to other Ceanothus species comes from work on free-living microorganisms 

and legumes. Calcium is required by most free-living N2-fixing microorganisms and 

is essential for fixation in leguminous hosts.2 A deficiency in phosphorus may inhibit nodulation, and a low soil pH has been shown to decrease the effectiveness 

of the legume endophyte (see footnote 3). The micronutrients molybdenum, cobalt, 

and nickel are necessary for N2 fixation in legumes and may be required as well 

by nonleguminous plants (Silvester 1977). Nickel is essential for the synthesis of 

hydrogenase in Rhizobium japonicum and presumably is required for this enzyme 

to function in nonleguminous plants (see footnote 3). High levels of aluminum and 

manganese may be toxic to either the endophyte or host. 

Environmental extremes may influence both the endophyte and the host and will 

directly affect N2 fixation. Variables such as soil temperature, soil moisture, and 

light affect physiological processes in C. velutinus (McNabb and Cromack 1983, 

Wollum and Youngberg 1969). Soil temperature affects both the number and size of 

nodules formed on C. velutinus (Wollum and Youngberg' 1969). Nitrogen-fixation 

rates in C. velutinus can decrease following decreases in soil temperature, soil 

moisture, and light (McNabb and Cromack 1983). Kummerow and others (1978a) 

found fewer healthy nodules on Ceanothus plants when the soil was dry than when 

the soil was moist. Wollum and Youngberg (1969) found maximum nodulation on C. 

velutinus when the soil temperature was 23-26 OC (73-79 OF). 

Age of seedlings, site preparation, and disturbance also affect nodulation and N2 

fixation. Kummerow and others (1978a) found nodules on only 4 of 100 6- to 

8-month-old Ceanothus tomentosus seedlings that had germinated following a fire 

in the Tecate Mountains, although they did find nodules consistently on 5- to 

10-year-old shrubs of C. greggii. Youngberg (1966), however, notes maximum 

nodulation following logging and burning, provided the age of the timber is 100 

years or less. He attributes high nodulation to increased light intensity on the site 

following logging and to heat scarification of Ceanothus seeds by fire. He also 

hypothesizes that poor nodulation on older sites could result from a lack of the endophyte when a site had been dominated by conifers for many years. Nodulation 

and the general health of the plants declined after several years, probably because 

of canopy closure and the intolerance of Ceanothus to shade. 

~~ ~ ~ ~~~ 

9 Personal communication, H. Evans, Botany Department, 

Oregon State University, Corvallis, OR 97331. 

25 
Photosynthates produced by the host and transported to the roots supply the 

necessary carbohydrates to the nodular endophyte (Paul and Kucy 1981). 

Rodriguez-Barrueco and others (1970) suggest that when N is added to the soil in 

the form of fertilizer, the host plant grows faster and utilizes surplus carbohydrates, 

thus denying a carbon supply to the endophyte and reducing N2 fixation. Youngberg 

(1966) states that C. velutinus produces less photosynthate under shade conditions 

and, consequently, has fewer nodules and lower N2 fixation. 

Soil chemistry, including such factors as cation exchange capacity, base saturation, 

organic matter content, and ion concentrations, influences the endophyte as it lives 

in the soil and directly influences the growth of the host plant and degree of 

nodulation (Scott 1973). White (1967) notes an absence of nodules on C. cuneatus 

roots in plants growing in serpentine soil. This may be because of ion imbalance, 

especially of calcium, magnesium, nickel, and chromium. It is not known if the ions 

affect the populations of the endophyte or the actual nodulation process. 

Nodulated Ceanothus plants are also commonly mycorrhizal. It has been demonstrated that when legumes are mycorrhizal there is an increase in nodulation and 

N2 fixation (Carling and others 1978). Rose and Youngberg (1981) found a similar 

synergism in C. velutinus. Nodulated Ceanothus with mycorrhizal fungal partners 

produced three times more nodular tissue and fixed about twice as much N2 per 

plant as did nodulated plants without mycorrhizae. Increased N2 fixation both in 

legumes and actinorryhizal plants is attributed to increased nutrient uptake and 

healthier plants producing more photosynthates than nonmycorrhizal nodulated 

plants (Carling and others 1978, Gardner and others 1984, Rose and Youngberg 

1981). 

Methods for Measuring 

N2 Fixation 

The methods for measuring N2 fixation in both the field and laboratory include 

acetylene reduction, natural isotope (15N) tracer studies, isotope (15N) enrichment 

studies, N-accretion studies, and analysis of N-transport products in xylem sap. 

Each of these methods has a bias and built-in assumptions that may or may not 

be valid for an individual study and may account for much of the variation in 

reported N2-fixation rates and N accumulation. 

Sources of errors in determining N2-fixation rates arise from taking too few samples 

and from making wide extrapolations from one sample at one time. The difficulty of 

estimating nodule biomass accurately, seasonal variations in nodule biomass, 

short-term variations in nodule efficiency and activity (due to changing environmental conditions), and differences among sites add to the problem of accurately quantifying N2-fixation rates. 

Rates of N2 fixation vary by Ceanothus species, site, age of the plants, season, and 

light regime (McNabb and Cromack 1983, McNabb and Geist 1979). Some of the 

rates of N2 fixation (or accretion) that have been estimated in field studies are summarized in table 1. 

The participants in this session discussed at length the most important methods for 

measuring N2 fixation and changes in soil N levels in laboratory and field studies. 

This discussion focused on the appropriate or potential uses, the assumptions required for drawing conclusions, and the potential sources of error for each method. 

A brief description of each method and a summary of the points brought up in the 

discussions follow: 

26 
Table 1--Estimated rates of N accretion or N2 fixation on forest and chaparral 

sites in the Western United States 

~~ 

Ceanothus species 

and location 

Accretion 

or fixation Stand age Source Method 

kg/ha/yr years 

Ceanothus velutinus, 

H. J. Andrews Experimental 

Forest, Oregon 3 42-48 0 and 12 Binkley and others chronosequence 

3 95-100 1982 

C. velutinus, H. J. Andrews 

Experimental Forest, Oregon McNabb & Cromack 

1983 

acetylene 

reduction 

3 101 17 

C. velutinus, Cascade 

Range, Oregon '1 56 0-1 5 

84 

Youngberg and 

others 1979 

accretion 

C. velutinus, Cascade 

Range, Oregon 39 4 0-40 2-33 

39 < 0-46 (96) 

Bin kley unpublished 

data, Zavitkovski and 

Newton 1968 

chronosequence; 

accretion 

C. velutinus, Cascade 

Range, Oregon 9 72-1 08 0-1 0 Youngberg and 

Wollum 1976 

accretion 

C. sanguineus, Vancouver 

Island, British Columbia 3 24-50 

11 45-80 

10 Binkley and Husted 

1983 

accretion, based 

on spatial 

variation 

C. greggii var . perplexans, 

San Diego County, 

California 30.1 25 Kummerow and others 

1978 

acetyl en e 

reduction 

C. crassifolius, San Dimas 

Experimental Forest, 

Southern California 3 28 

2/76 

0-1 3 Zinke 1969 accretion 

3 Soil N accretion. 

2/ Ecosystem N accretion (soil, litter, biomass). 

2/ N2 fixation in field. 

9 Highest number is average over first 10 years. 

27 
1. Acetylene reduction.-In the acetylene reduction method, nodule tissue is incubated in an acetylene-enriched atmosphere, and the amount of ethylene evolved 

is assayed as an indirect measure of nitrogenase activity in the nodules (Postgate 

1972). The efficiency of the reduction reaction may sometimes be considerably less 

than the theoretical ratio of 0.3 moles of N production for each mole of ethylene 

evolved (Bergersen 1980, Hardy and others 1968). Additional sources of error occur 

when this method is used to estimate seasonal or annual N2-fixation rates per unit 

area in the field. To develop reasonable estimates using this approach, investigators need estimates of the current age, structure, and vigor of the stand. By measuring levels of environmental regulating factors such as light and moisture in the 

field, laboratory data on responses of fixation (acetylene reduction) to environmental conditions can be incorporated into these models. 

2. Accretion.-Estimates of soil N accretion over time can be made by successive 

measurements of soil N levels on one or more sites (Youngberg and Wollum 1976) 

or in experimental plots in stands of pure species composition (Zinke 1969). Many 

years are obviously required to obtain meaningful results with this approach, and 

the accuracy of estimates relies heavily on the accuracy of initial estimates of N 

pools and soil N. 

3. Chronosequence.-Another method for studying accumulation of soil N over 

time is to measure the soil N levels on sites with a range of ages following the 

establishment of Ceanothus (Binkley and others 1982, Zavitkovski and Newton 

1968). In this approach, the assumptions must be made that (1) baseline levels of 

soil N are the same on all sites and (2) historical patterns of N2 fixation have been 

the same on all sites. Even if sites are selected very carefully, differences in site 

characteristics (such as micronutrient levels) and in patterns of vegetation development or of nodulation are likely to make these assumptions questionable. Violation 

of these two assumptions can introduce substantial error into estimates of N accumulation made by this method. 

It is important, furthermore, to recognize that both chronosequence and accretion 

studies yield only estimates of changes in soil N levels over time, not rates of current or past N2 fixation. These methods do not generally take into account N losses 

caused by plant uptake, leaching, and denitrification, nor do they allow compartmentalization of input sources such as rainfall, lightning, and nitrification. 

Therefore, estimates of N balance made by these methods do not represent fixation rates unless corrections can be made for other inputs and outputs of N in the 

ecosystem. If inputs and outputs can be quantified independently these methods 

might allow N2 fixation to be estimated with reasonable accuracy. 

4. Natural isotope distribution.-In most soils, the ratio of 15N to 14N is higher 

than it is in the atmosphere. The basis for the natural isotope discrimination 

method for estimating N2 fixation is that the ratio of these isotopes in plant tissues 

should reflect the source of the plant’s N. A plant that obtains most of its nitrogen 

through nodular fixation of N2 therefore, has a ratio close to that in the atmosphere, 

whereas plants that do not fix N2 have ratios closer to that in the soil. Theoretically, 

the ratio, 15N:14N (expressed commonly as atom percent excess of 15N), can be 

partitioned to yield an estimate of what proportion of a plant’s N has been derived 

from the atmosphere (Bergersen 1980, Delwiche and others 1979, Silvester 1983). 

28 
This method permits the investigator to follow N through an ecosystem or through 

a plant. A potential problem with this approach is that the within-pool variation in 

various ecosystem components (such as soil and plant tissue) may be high enough 

to make detection of differences among species, among ecosystem components, or 

among treatments difficult. The advantages of this method for ecosystem studies 

are that it gives integrated values of N2 fixation over time, it can be used where 

nodules are difficult to locate or to excavate, and measurements are made on easily obtained materials (leaves, stems, soil). The method has been used successfully 

to estimate N2 fixation by mesquite and other species in desert ecosystems 

(Shearer and others 1983) and to detect differences in the atom percent excess of 

l5N between presumed N2-fixing and nonfixing plants in a wide range of northern 

California ecosystems (Virginia and Delwiche 1982). 

5. 15N isotope enrichment.-In this method, the nodules or roots are placed in an 

atmosphere that has been enriched in known amounts with 15N2 (Bergersen 1980, 

Silvester 1983). The atom percent excess of 15N in the samples (plants, nodules, 

roots) following treatment can provide an estimate of the amount of N2 fixed during 

the incubation period and (for whole plants) of the distribution and biochemical fate 

of that N within the plant. This method allows direct comparisons to be made 

among fixing and nonfixing plants. Disadvantages include the high costs of isotopic 

N and of chemical analysis. The same problems of high within-pool variability that 

were discussed before can occur, especially in field studies, and laboratory studies 

require disturbance of living plant material as well as its monitoring in a somewhat 

unnatural environment. Despite potential shortcomings, this technique has provided 

extremely useful information in a large number of studies. It was used by Delwiche 

and others (1965) to estimate rates of N2 fixation for 12 Ceanothus species that occur on forest and chaparral sites in California. 

6. Xylem sap.-In both legumes and nonlegumes, certain N-transport compounds 

found in the xylem serve primarily as carriers for symbiotically fixed N, while other 

compounds are carriers for N obtained from the soil solution (Dixon and Wheeler 

1983, Pate 1980). In soybeans, for example, the abundance of ureides (relative to 

that of nitrate-N) in xylem sap decreased from 94 percent to 9 percent as the proportion of plant N derived from N2 fixation decreased from 100 percent to 1 percent 

(Herridge 1982). In this same study, a positive relationship was apparent between 

the concentration of ureides in the xylem and the rate of N2 fixation measured by 

acetylene reduction. The major transport compounds for symbiotically fixed 

nitrogen are species specific (Pate 1980). Citrulline appears to dominate in alder 

and asparagine in Ceanothus (Dixon and Wheeler 1983). Relationships could 

theoretically be determined between rates of N2 fixation and concentrations of Ntransport compounds in the xylem by obtaining independent estimates of N2 fixation using acetylene reduction or 15N enrichment techniques. Although this approach is still in the early stages of development, it shows particular promise for 

studies of deep-rooted woody species such as Ceanothus, because it has potential 

for permitting N2 fixation to be estimated without access to the rooting zone of the 

plants. 

. Additional details, and a wealth of references, on methods of evaluating N2 fixation 

can be found in Bergersen (1980) and in Silvester (1983). 

29 
Research Needs The following needs for future research on interaction between Ceanofhus species 

and the soil system were identified: 

1. Identify the factors that influence nodule establishment and development. 

a. Determine factors that influence the presence and efficacy of the 

endophyte. 

b. Quantify the influence of microenvironmental parameters (such as water, 

light , temperature). 

c. Quantify the influence of soil chemistry characteristics, such as soil pH, 

calcium, and micronutrient levels. 

2. Identify the factors that limit N2-fixation rates. 

a. Determine the importance of genotypic variability of hosts and endophytes. 

b. Measure effects of factors such as soil moisture, soil temperature, plant 

moisture stress, light, and photosynthetic rates. 

3. Identify the factors that affect N-cycling rates. 

a. Determine whether or not non-N2-fixing plants cycle as much N in litterfall 

as do plants that fix N2. 

b. Determine the importance of abiotic factors and site characteristics in 

regulating N cycling. 

c. Determine the influence of Ceanofhus on pathways and rates of N cycling. 

4. Quantify the changes in soil organic matter that accompany N accretion, and 

determine the effects of soil organic matter on soil moisture infiltration, 

moisture retention, soil aggregation, soil aeration, and overall soil structure. 

5. Investigate litter decomposition patterns. 

a. Quantify rates. 

b. Determine the roles of microorganisms and of macrofauna, such as 

earthworms, in these patterns. 

30 
5. Physiological Ecology of Ceanothus and Associated 

Conifer SpeciesThe presence of Ceanothus on a site can affect many aspects of the conifer growth 

environment other than nitrogen availability and nutrient cycling. Effects of 

Ceanothus on environmental factors-such as light, water, and temperature-as 

well as production of allelopathic substances can potentially affect the growth and 

survival of associated conifers. The relative abilities of conifers and Ceanothus 

species to exploit resources and to tolerate resource depletion are determining factors in dynamic interactions in the vegetation. This section considers effects of 

Ceanothus on environmental factors, effects of environmental conditions on conifers 

and Ceanothus species, and the implications of these effects for shrub-conifer 

interact ions. 

Effects of Ceanothus 

on the Conifer 

Growth Environment 

Light.-Many Ceanothus species have dense canopies. Few data are available, 

however, on the actual light levels under Ceanothus canopies. Petersen (1980) used 

ozalid paper and measured 30 percent of full sunlight (integrated over the whole 

day) at the bottom of the live canopy in 5- to 10-year-old C. velutinus stands in the 

Cascade Range. Conard and Radosevich (1982a) report 7 percent of full sunlight at 

midday at the soil surface under 35- and 50-year-old C. velutinus in the Sierra 

Nevada. McPherson and Muller (1967) used a hand-held light meter to find values 

of 200 to 4,000 foot-candles under C. cuneatus canopies in Santa Barbara County, 

California. 

Soil temperature.-Soil-surface temperatures in summer under C. velutinus 

canopies in the Cascade Range were 11 to 33 OC (20 to 60 OF) lower than 

temperatures in adjacent unshaded areas (Scott 1969, Youngberg 1966). The 

average soil-surface temperatures in August as measured in open areas by Scott 

(1969) frequently exceeded 65 OC (150 OF). Temperatures this high have been 

reported to cause substantial seedling mortality in many conifer species (Hare 

1961, Silen 1960) and could be expected to inhibit survival of natural germinants in 

unshaded microsites. 

Midsummer soil temperatures at the 15- and 45-centimeter (6- to 18-in) depths in 

areas where C. velutinus and Arctostaphylos patula had been removed were 

generally 1 to 4 OC (1.8 to 7.2 OF) higher than temperatures where the shrubs had 

not been removed (S. Conard, unpublished data). Mean temperatures in late July 

at the 15-centimeter (6-in) depth in this study were 17 OC (63 OF) in the open and 

15 OC (60 OF) under the canopy. 

3 Participants in this discussion group were: Ned Dimock, lvend 

Holen, Denis Lavender, Bill Lopushinsky, Dave McNabb, Pete 

Owston, Terry Petersen, Greg Prull, Steven Radosevich, Bob Tinnin, Bob Wagner, Jack Walstad, and Diane White. The discussion leader was Susan Conard. 

31 
Although high soil-surface temperatures may inhibit survival of conifer germinants, 

there is no evidence that they have negative effects on planted seedlings. Two 

potential benefits to conifers from slightly higher temperatures in the rooting zone 

are that root growth could begin earlier in the spring in open areas and that higher 

soil temperature could enhance the ability of roots to remove water from the soil. 

These benefits could be compounded by the soil where shrubs have been removed 

having substantially more soil moisture below the top 15 centimeters (6 in) than the 

soil where shrubs are present. 

Little or no information is available on the influence of other Ceanothus species on 

soil temperatures or on the relative ecological importance of small differences in 

rooting-zone temperature on conifer growth. Without additional research, it is impossible to determine the ecological importance of small temperature differences in 

the rooting zone of planted or naturally established conifers. Routine installation of 

thermocouples or thermistors in experiments measuring soil moisture with moisture 

blocks could provide a great deal of soil temperature information at low cost. 

Air temperature.-Conard and Radosevich (1982a) observed higher air 

temperatures inside live Ceanothus canopies than in open areas. They consistently 

observed minimum temperatures 2 to 3 OC (3 to 5 OF) lower and maximum 

temperatures 2 to 3 OC (3 to 5 OF) higher under a C. velutinus canopy than at the 

same height outside the canopy or at the same height (50 cm; 20 in) in the open. 

This relationship is probably a function of reradiation from stems and a lack of air 

mixing within the canopy. The relationship is probably dependent on canopy structure and would not be expected to be true for all species of Ceanothus. Further 

research is needed to: (1) determine how common these effects are within shrub 

canopies, (2) evaluate the effects of factors such as time of year and aspect, and 

(3) determine what influence (if any) such temperature differences might have on 

conifer growth or survival. 

Soil moisture.-Several studies have measured soil moisture under C. velutinus 

canopies and compared it to soil moisture in areas where shrubs were removed or 

where shrubs were absent. Soil moisture in the top 15 centimeters (6 in) remained 

higher throughout the season under C. velutinus canopies than in the open (Scott 

1969, Youngberg 1966). In these studies, however, “open” areas had dense covers 

of grasses and other herbaceous vegetation that typically have shallow rooting 

systems as compared to shrub species. Petersen (1980) created open areas in C. 

velutinus brushfields by removing shrubs and herbaceous vegetation with herbicides. In his study, soil moisture at 10 centimeters (4 in) was depleted more 

rapidly by shrubs than by herbaceous vegetation on one site. On another site, soil 

moisture depletion by shrubs and herbaceous vegetation at 10 centimeters was 

comparable. At 40 centimeters (16 in), shrubs depleted soil moisture more rapidly 

than did herbs on both sites. On one site, soil water potentials under herbaceous 

vegetation reached a minimum of about -8 bars for the season; water potentials 

under shrubs had decreased to less than -15 bars by late July. At 100 centimeters 

(39 in), soil water potential stayed greater than about -2 bars under herbaceous 

vegetation and was decreased to -15 bars by mid-July to mid-August where shrubs 

were present. Soil moisture showed no seasonal decrease at any depth where 

measurements were made if both shrubs and herbs were controlled. 

32 
Conard and Radosevich (1982a) report similar responses in soil moisture levels 

under C. velutinus canopies and in treatments where shrubs were removed in the 

Sierra Nevada. Scott (1969) also reports substantially lower minimum soil water 

potentials under C, velutinus (about -12 bars) than in the open under herbaceous 

vegetation (about -8 bars) at a 70-centimeter (24-in) depth. C. prostratus has also 

been shown to deplete soil moisture to approximately -15 bars by late summer at 

depths from near the surface to 45 centimeters (18 in) (Tappeiner and Helms 1971). 

Although specific information on many Ceanothus species is lacking, the data 

presented above for C. velutinus and C. prostratus suggest that these species growing on forest sites can decrease soil water potentials to near -15 bars by late summer in the rooting zone of young conifers. Comparisons with areas where Ceanothus is not present must be made carefully as results can vary widely depending 

on what other vegetation is present on the site or on whether soil conditions in 

areas being sampled are comparable. 

Humidity.-The presence of transpiring Ceanothus crowns causes substantial local 

increases in humidity and decreases in wind velocity. The net result of these effects is a decrease in evaporative demand. Conard and Radosevich (1982a) and 

Petersen (1980) document 120- to 180-percent increases in evaporation rates 

(measured with Pich6 evaporimeters) on C. velutinus sites when either shrubs or 

shrubs and herbs had been removed. The participants were aware of no similar 

studies on other Ceanothus species. 

Allelopathy.-Tinnin and Kirkpatrick (in press) evaluate the allelopathic potential of 

water extracts of recent litterfall from several important shrub species in southwestern Oregon. In laboratory bioassays, C. integerrimus and C. velutinus reduced 

growth of Cucurbita by 30 percent and 60 percent, respectively. Ceanothus 

velutinus extracts also reduced root elongation of Douglas-fir germinants. Experiments in natural soils are in progress. Conard (1985) found that water extracts 

of fresh C. velutinus foliage also cause significant inhibition of radicle elongation 

in white fir germinants. 

Rose and others (1983) found that water-soluble extracts of C. velutinus litter had 

varied effects on the growth of several mycorrhizal fungi that are found on conifers. 

All concentrations (parts per thousand, per hundred, per ten) stimulated the growth 

of Cenococcum geophilum. High concentrations (parts per ten) of extract inhibited 

the growth of Laccaria laccata. C. velutinus litter leachate also reduced mycorrhizal 

form at ion on Doug las-f i r seedlings. 

No studies to date have demonstrated the allelopathic effects of Ceanothus species 

on conifers in the field. Because of the observed effects on root growth and mycorrhizal formation in laboratory studies, allelopathy might be expected to be most important in situations where water or minerals are limiting and where the inhibition 

of root elongation significantly decreases eff icienzy of water or mineral uptake. 

33 
Effects of Environ- mental Conditions 

on Conifer Growth 

The effects of Ceanothus on conifer growth are a function of how well the conifers 

are able to establish, survive, and grow under the modified environmental conditions established by Ceanothus and of how well the conifers can compete for 

potentially limiting resources such as light and water. 

Factors that can affect growth of conifer species include the photosynthetic surface 

area of individual plants and the photosynthetic efficiency of the tissues. Under 

potentially I im it i ng environmental conditions, responses of photosynthesis to I ig ht, 

temperature, and water availability can greatly influence seasonal growth in the 

field. Relative growth rates can provide valuable information about responses to different environmental conditions. 

Light.-Although most conifers do not require full sunlight for maximum photosynthesis of individual branches or of seedlings, the light saturation level of a mature 

tree can vary considerably as a function of crown structure. Most pine species, 

however, require close to full sunlight for maximum photosynthesis. Light requirements that have been reported for maximum photosynthesis of conifers range 

from about 2,000 to 2,500 foot-candles (-400 to 500 pE m-2 sec-1) for seedlings of 

Douglas-fir and western hemlock (Brix 1967, Krueger and Ferrell 1965, Krueger and 

Ruth 1969). In contrast, light saturation is around 1 000 pE m-2 sec-1 for individual 

fascicles of ponderosa pine (Coyne and Bingham 1982), and 3,500 and >9,500 

foot-candles (about 700 and >1 900 pE m-2 sec-I), respectively, for needles and 

seedlings of loblolly pine (Kramer and Kozlowski 1979). 

Minimum light requirements for establishment and survival are considerably lower, 

although little quantitative information is available (Atzet and Waring 1970). Light requirements may be particularly important when the requirements of the conifer 

species differ from those of associated noncrop species with which they are competing. For example, Conard and Radosevich (1981) found that white fir reached 90 

percent of maximum photosynthesis at 0.53 mmol m-2 sec’ (1 mmol =IO3 HE), 

compared with 0.71 mmol m-2 sec-1 for Ceanothus velutinus. This ditterence in light 

requirements fits in well with the ability of white fir to survive under the shrub 

canopy and eventually to overtop and shade out the Ceanothus. 

Several authors (Baker 1949, Minore 1979, Waring 1970) have ranked the shade 

tolerance of conifers. Of the western conifers, western hemlock and western 

redcedar are among the most shade tolerant, most true firs are intermediate, and 

pines have relatively low shade tolerance. Despite ranges in tolerance (ability to 

survive and grow at low light levels), both greenhouse and field studies suggest 

that growth of a wide range of conifer species increases with increasing light up to 

70 to 100 percent of full sunlight. Species studied in the field (Emmingham and 

Waring 1973) have included Douglas-fir, white fir, and red fir. 

With field studies it is difficult to separate effects of light level on photosynthesis 

and growth from effects of factors such as water availability. In greenhouse studies 

on Doug las-f i r, i ncense-cedar, Port-Orford-cedar, ponderosa pi ne, Jeffrey pi ne, 

Monterey pine, and tamarack, only growth of Port-Orford-cedar, Jeffrey pine, and 

Douglas-fir showed clear evidence of light saturation-at about 65 to 70 percent of 

full sun (Baker 1945). 

34 
It is frequently difficult to extrapolate lab results to field situations where adaptations of foliage to light may vary and environmental factors such as temperature 

and water stress may limit photosynthesis. Even under field conditions where light 

may be expected to limit photosynthesis, other limiting factors may be more important. For example, increasing available soil moisture may stimulate photosynthesis 

and growth, while increasing light availability may have little or no effect if water remains limiting. In terms of management, the most limiting factor or factors must be 

identified and ameliorated for a substantial growth response to occur. In the 

absence of other limiting factors, however, we would expect conifer saplings to 

show significant increases in growth anytime light is increased from low levels (less 

than 50 percent full sun) to almost full sunlight. 

The light levels a seedling has experienced previously will also precondition it and 

will affect its response to manipulation of light intensity. Reduced light decreases 

canopy volume and vigor and alters the structure, photosynthetic efficiency, and 

responses of the photosynthetic apparatus and stomata to low light intensities. 

Because of these changes, suppressed conifers may require a period of adaptation 

following sudden exposure to full sun, primarily because shade leaves have poorer 

ability to control water loss than do sun leaves. Sudden exposure may result in 

mortality, nearly total loss of current needles, or severe reduction in height and 

diameter growth for one or more years. Plants that survive can be expected to 

recover once a full complement of sun-adapted foliage is developed. Responses of 

this type have been observed for western hemlock (Keller and Tregunna 1976), 

white fir (Conard and Radosevich 1982a), and Douglas-fir (Petersen 1980). The 

severity of exposure shock will vary among species and within species among 

habitats. Assuming that exposed plants survive, then long-term growth response to 

overstory removal should be positive. 

The decreased radiation load and higher relative humidity under shrub canopies 

appear to aid the establishment of seedling conifers in both C. prostratus (Tappeiner and Helms 1971) and C. cordulatus (S. Radosevich, unpublished data). Based on the effects of C. velutinus on evaporative demand and light intensity, similar 

factors might explain differences in natural seedling densities of Douglas-fir that 

have been observed under and outside C. velutinus canopies (See section 5 for additional detail on these studies). 

Soil moisture.-Both field and laboratory studies have shown that soil water potentials between -1 and -5 bars are sufficient to limit photosynthesis in many conifer 

species (Havranek and Benecke 1978, Zavitkovski and Ferrell 1970). In a laboratory 

study of five western conifers (Lopushinsky and Klock 1974), transpiration rates 

decreased to 50 percent of maximum at soil water potentials ranging from -4 to -8 

bars. At -10 bars, transpiration rates of grand fir and ponderosa pine were about 35 

and 12 percent of maximum values, respectively. At high vapor pressure deficits 

between leaf and air, stomata will close at even higher soil water potentials than indicated by these studies. 

Because Ceanothus species can decrease soil moisture to below these levels by 

midsummer, conifers growing with Ceanothus can be expected to have transpiration 

and photosynthesis rates below their potential maxima for some of the growing 

season on many sites. Low soil and plant water potentials would also be expected 

to reduce conifer root growth. 

35 
Physiological 

Responses of 

Ceanothus Species 

to Environmental 

Conditions 

In general, Ceanothus species maintain high stomatal conductances and high 

photosynthesis rates at substantially lower plant and soil water potentials than do 

associated conifers. Photosynthesis rates of C. integerrimus, C. cordulatus, and C. 

velutinus were near their seasonal maxima at plant water potentials at least as low 

as -25 to -30 bars, -24 bars, and -20 bars, respectively, in a study in the central 

Sierra Nevada (Lanini 1980). Conard and Radosevich (1981) found that photosvnthesis and leaf conductance of C. velutinus remain at maximum rates even in late 

summer at the lowest plant water potentials observed (-18 bars). Other studies 

(Marshall and Waring 1984) have found that C. velutinus in the Oregon Cascade 

Range has extremely high stomatal conductances and poor stomatal control. 

Stomatal closure has been reported to occur at leaf water potentials of -30 and -20 

bars for C. thyrsiflorus and C. incanus, respectively (Barnes 1981). These species all 

occur in forest habitats. For Ceanothus species of drier chaparral habitats, 

tolerance to low water potentials may be even greater. Stomatal closure has been 

reported at water potentials of -55, -45, and -55 to -60 bars for C. ferrisae, C. 

ramulosus, and C. greggii, respectively (Barnes 1981, Poole and Miller 1975). 

Ceanothus species in general are capable of maintaining high photosynthesis and 

transpiration rates under conditions when photosynthesis of conifers will be severely limited. This is demonstrated by Conard and Radosevich (1981) for C. velutinus 

growing with white fir in the Sierra Nevada. Copeland (1932) estimates that C. 

velutinus and C. cordulatus in the northern Sierra Nevada transpire approximately 

0.6 meters (2 ft) of water per unit leaf area over the growing season. He estimates 

that transpiration of Castanopsis sempervirens, Quercus vaccinifolia, and Q. kelloggii 

was about half that of the two Ceanothus species. Ceanothus species also have a 

high light requirement for maximum growth (Schlesinger and others 1982) and 

photosynthesis (Conard and Radosevich 1981). This characteristic might be expected for early seral species. 

Conclusions.-Ceanothus species can decrease light intensities to levels well 

below those required for maximum growth of conifers. Thus, one would expect 

reduced growth of conifers under Ceanothus canopies even in the absence of other 

limiting factors. On many sites where Ceanothus and conifers are growing together, 

however, soil moisture depletion is probably the major factor limiting conifer 

photosynthesis except, perhaps, during the early part of the growing season. 

Ceanothus may have a competitive advantage because it can absorb water from 

relatively dry soil. In spite of high internal water stress, Ceanothus can maintain 

open stomata and continue photosynthesis. Conifers are less able to do so. The 

result is that Ceanothus species have the potential to compete strongly with conifers and to reduce conifer growth. 

Levitt (1980) suggests that at high stress, users tend to revert to conserver 

strategies. It appears that stress levels in forest systems are rarely high enough to 

induce this for Ceanothus. Even if it does occur, little benefit could be expected to 

accrue to the less tolerant conifers. 

The potential benefits to conifers from N2 fixation by Ceanothus species are 

discussed in section 4. These and other benefits (such as amelioration in seedling 

environment and improved wildlife habitats) will need to be balanced against potential growth losses in making management decisions. The duration of conifer growth 

suppression could vary widely from site to site depending how long it takes for 

conifers to achieve dominance and begin to suppress associated Ceanothus. 

36 
Research Needs This discussion group identified 12 areas where further research is needed: 

1. Effects of canopy structure of different Ceanothus species on environmental fac-, 

tors such as light penetration, soil and air temperatures, humidity, and surface soil 

moisture need to be determined, and influences of season, aspect, and slope need 

further study. 

2. Descriptions are needed of the development over time of vertical and horizontal 

structure of Ceanothus canopies (individual shrubs and com m u n it ies) including 

biomass and leaf area. 

3. Patterns of light distribution under canopies need to be quantified using techniques such as fisheye lens photography, quantum (PAR) sensors, or correlations 

with canopy structure, biomass, leaf area, and canopy volumes. 

4. Studies are required on the effects of light levels, such as those found under 

Ceanothus canopies, on the productivity of conifers at various stages of community 

development. 

5. Importance of the allelopathic potential of Ceanothus species in field situations, 

including the influence on conifer growth and rates of N2 fixation, should be investigated further. 

6. Effects of environmental modification by Ceanothus species on their own ability 

to fix N2 need to be quantified. There is some evidence that dense stands of C. 

velutinus may reduce soil moisture to levels that inhibit symbiotic N2 fixation. This 

topic was discussed in more detail in section 4. Effects of canopy structure and 

distribution on factors such as soil temperature might also be important here. 

7. Correlations should be developed that would permit information on photosynthesis and plant water potential of Ceanothus species to be used to estimate or 

predict N2-fixation rates under varying enviromental conditions. 

8. More information is needed on water utilization by both Ceanothus and conifer 

species and on distribution of roots horizontally and by depth. Seasonal patterns of 

water use at different depths can provide a better understanding of the degree of 

synchrony of water demands by shrubs and conifers and how this affects the intensity of competition. Better information on root distribution could aid greatly in interpreting studies that are concerned with soil moisture depletion, plant-water relations, and shrub-conifer interactions. 

9. Relationships need to be established between canopy volume, biomass, leaf 

area, and spatial distribution of Ceanothus species and their effects on soil 

moisture regimes. 

10. More field data are needed on relationships between environmental moisture 

deficit, plant moisture stress, photosynthesis, and productivity for conifer species, 

including effects of water stress in the field on the initiation of conifer dormancy. Information on young planted seedlings of the major commercial species would be 

particularly beneficial when combined with information from items 1, 2, and 9. 

37 
11. Additional research is needed on the relative strength of interspecific competition between Ceanothus and conifer species and intraspecific competition among 

conifers in various situations. These topics are discussed more thoroughly in section 6. 

12. A great deal of research relevant to the physiological ecology of Ceanothus 

species in forest habitats has centered on C. velutinus. More information on other 

common forest species such as C. integerrimus, C. cordulatus, C. prostratus, C. 

sanguineus, and C. thyrsiflorus would be extremely valuable. 

6. Effects of Ceanothus on Forest Productivity and 

Conifer Growt hw 

Effects of Ceanothus 

on Conifer 

Establishment 

In many ways, this discussion was the focus of the workshop. The discussion provided a forum for integrating some of the basic information presented at earlier 

sessions, and studies that have attempted to quantify responses of conifers in relation to the presence of Ceanothus species were discussed . The major topics were 

the effects of Ceanothus on (1) conifer establishment, (2) early growth of conifers, 

and (3) site quality. 

Several studies have documented better initial establishment of conifer species 

under Ceanothus canopies than in the open. Scott (1969) reports 53-percent stocking of Douglas-fir in C. velutinus clumps in the Cascade Range in Oregon, as compared to 36-percent stocking in open areas on the same site. Dyrness and 

Youngberg (1966), working in an area where canopy coverage of C. velutinus was 

33 percent, found that 66 percent of the ponderosa pine seedlings on the site are 

associated with Ceanothus clumps. Tappeiner and Helms (1971) report better survival of Douglas-fir and white fir natural regeneration in C. prostratus mats than in 

several other microsites in the Sierra Nevada; Lanini (unpublished data) observed 

improved survival of planted ponderosa pine, sugar pine, and Douglas-fir seedlings 

in association with C. cordulatus and other shrub species. Show (1924) also reports 

improved survival of planted ponderosa pine, sugar pine, and giant sequoia as 

shading by brush increased (in mixed brushfields containing Ceanothus velutinus, 

Arctostaphylos patula, and Castanopsis sernpervirens). Show concludes that the 

benefits of shading are greatest on poor sites, in dry years, and with small planting 

stock. Wahlenberg (1930) reports first season survival of 89 percent for planted 

ponderosa pine under C. velutinus canopies and 53 percent in the open in the 

Rocky Mountains in Montana. He attributes this, in part, to higher humidity, lower 

evaporative demand, cooler soil temperatures, and higher surface soil moisture 

under the shrub canopies. 

3 All workshop participants were members of this discussion 

group. Discussion leader was Bill Stein. 

38 
In the only experimental study we are aware of, Zavitkovski and others (1969) 

planted seedlings of four conifer species under live C. velutinus, under dead C. 

velutinus, in openings, and on freshly burned areas. All species survived best in 

freshly burned areas. Survival in the other three treatments was equal for 

ponderosa pine and Douglas-fir. Western hemlock had its lowest survival in openings, and noble fir survival was lowest under live and dead Ceanothus. Clearly, different species can be expected to respond differently to the presence of 

Ceanothus. These studies suggest that where Ceanothus benefits conifer survival, 

results may apply both to natural regeneration and to planted seedlings. 

Causes for the observed effects of Ceanothus species on initial survival may vary. 

As discussed in a previous section, summer soil temperatures and evaporative demand are frequently lower under Ceanothus canopies than in the open. There is 

also evidence that surface soil moisture remains higher underneath the canopy 

than where Ceanothus is absent (herbaceous vegetation is usually present in the 

latter areas). These factors are discussed more fully in section 5. Shrubs may provide protection from browsing on some sites, although there have also been reports 

of increased rabbit damage under Ceanothus canopies (Zavitkovski and others 

1969). Most reports discussed in this section were based on sample measurements 

rather than on controlled experiments, so causal relationships can only be 

postulated. Effects of brush on survival may be confounded in some of these 

studies with microsite differences between areas where shrubs established initially and areas where shrub establishment was poor (“open” areas). 

There is a lack of studies in which the effects of Ceanothus on the conifer environment have been specifically characterized and isolated from possible confounding 

factors. Some of the factors that need to be considered in design of research on 

this topic are: 

1. Effects of different levels (densities, sizes, cover) of Ceanothus need to be 

evaluated on comparable microsites. This requires a replicated, randomized design 

where levels of Ceanothus are manipulated experimentally. 

2. Animal damage and other sources of injury (for example, frost and winter desiccation) should be documented. 

3. Mechanical damage, such as that from snow, wind, or falling debris, should be 

documented if it occurs. 

Effects of Ceanothus There are two categories of research that have been conducted on the effect of 

on Early Growth of Ceanothus on early growth of conifers. The first involved studies in which the strucConifers . ture of Ceanothus stands was manipulated experimentally. The second group of 

studies measured conifer growth relative to varying levels or ages of naturally 

established, undisturbed Ceanothus. These two types of studies were further divided into those dealing with planted conifers and those concerned with natural 

regeneration. 

39 
Experimental Studies Several studies have demonstrated dramatic increases in height, diameter, and 

volume growth of conifers following release from C. velutinus. One of the earliest 

(Gratkowski and Lauterbach 1974) shows 6-year height growth in Douglas-fir to be 

170 to 255 percent that of untreated controls following aerial spraying of C. 

velutinus var. laevigatus. Petersen and Newton (1 982) measured 3-year responses of 

5- and 10-year-old Douglas-fir following control of C. velutinus or of both Ceanothus 

and herbaceous vegetation. Volume growth of 5-year-old trees was 258 and 415 

percent of controls, respectively, where shrubs only or shrubs and herbs were controlled. Results for 10-year-old trees were similar but less striking. Differences 

among treatments in the study by Petersen and Newton were still accelerating as 

of the last measurements. 

Studies by Weyerhauser Company in the Cascade Range in Oregon demonstrated 

similar increases in height and diameter growth following release of Douglas-fir 

from C. velutinus (Lauterbach 1967). Conard and Radosevich (1982a) report increases of 140 to 200 percent in white fir height growth relative to untreated controls 4 years after release from C. velutinus. The largest growth response was 

observed when shrubs were controlled and the conifers were artificially shaded for 

several years. Substantial growth check was observed in many released trees that 

were not shaded. Eight-year cumulative height growth of released trees in the Con 

ard and Radosevich study is 200 percent of controls, and growth of released trees 

is continuing to increase faster than growth of the untreated controls (S. Conard, 

unpublished data). Sanders (unpublished data) also reported greater height growth 

for unreleased trees than for released trees 1 year after manual release from C. 

velutinus. By the second year, released trees on 6 out of 7 sites were growing at 

prerelease rates, and after 4 years, released trees on all sites were growing 

significantly better than unreleased trees. 

In a study evaluating release of lodgepole pine from Ceanothus, no growth 

response was observed 2 years after treatment (R. Boyd, unpublished data). Other 

studies have shown, however, that growth reduction can occur for 1 or 2 years 

following treatment, so a delayed response may well occur. 

Only one study was discussed in which effects of site preparation methods on 

growth of planted conifer seedlings are evaluated. This was a study described 

earlier, where Zavitkovski and others (1969) planted seedlings in four site preparation treatments. For all four conifer species planted in these experiments, height 

growth was reduced if the species were planted in C. velutinus stands over 10 

years old. This corresponds to the age at which C. velutinus achieves maximum 

biomass. Based on 2-year height growth measurements, the effects of other 

treatments were inconclusive, except that ponderosa pine height growth was 

highest in fresh clearcuttings. 

Height growth of naturally established seedlings was substantially greater in the 

fresh clearcutting than under live C. velutinus; growth increases were 44 percent, 

66 percent, 116 percent, and 124 percent for western hemlock, Douglas-fir, 

ponderosa pine, and noble fir, respectively (Zavitkovski and others 1969). These 

values suggest that planted seedlings may experience planting check for the first 

two seasons following planting. It is unfortunate that longer term data are not 

available for these study sites. Height and diameter growth measurements on conifers 5 and 10 years following treatments could provide valuable information. 

40 
Sampling Studies 

The participants were aware of several current research projects in which effects of 

Ceanothus species on survival or growth of conifers are being investigated: 

1. A series of administrative studies by the USDA Forest Service in California, 

Idaho, and Oregon are comparing effects of various chemical and manual site 

preparation and release treatments on conifer growth, and survival and vegetation 

development on sites where Ceanothus species occur. The principal scientists are 

W. Stein and E. Dimock II, Pacific Northwest Forest and Range Experriment Station; f? McDonald, Pacific Southwest Forest and Range Experiment Station; and R. 

Boyd, Intermountain Forest and Range Experiment Station. 

2. An intensive study is being conducted through Oregon State University in the 

H.J. Andrews Experimental Forest, Willamette National Forest, to investigate the 

long-term effects of C. velutinus, at several densities, on conifer growth and on soil 

properties. This research is being conducted by P. Sollins, Oregon State University. 

3. A comparison of several release treatments on C. velutinus-dominated sites in 

western Idaho is being conducted through the Coordinated Research on Alternative 

Forestry Techniques and Systems (CRAFTS) program under the direction of S. 

Radosevich, Oregon State University. 

4. Several studies on conifer release have been started by scientists at 

Weyerhaeuser Company, Champion International, and Groundwork, Inc. 

Several researchers have reported greater height growth of young Douglas-fir 

under or at the edge of C. velutinus canopies than in open areas between shrubs 

(Horowitz 1980, 1982; R. Sanders, unpublished data; Scott 1969). The most 

dramatic results are those of Scott (1969), who reports Douglas-fir heights after 9 

years of 1.6, 1.2, and 0.8 meters (5.2, 3.9, 2.6 ft) under shrub canopies, at the edge 

of canopies, and in the open, respectively. Horowitz (1980, 1982) conducted a 

similar study on sites with C. velutinus in the hemlock and true fir zones in the 

Cascade Range. He reports slightly to significantly greater height growth by 

Douglas-fir in edge positions than by those either inside or outside shrub canopies 

on 60 percent of his hemlock-zone study sites. Growth was greatest in the outside 

position on 30 percent of the study sites. In the true fir zone, where C. velutinus 

was less abundant, conifer height growth decreased 40 percent from outside to inside shrub canopies, and growth in the edge position was intermediate. In this 

study, no correlation was observed between shrub cover and conifer growth. 

Horowitz (1982) measured conifer and shrub growth on forest sites proposed for 

aerial release treatments of 2,4,5-T in the Willamette National Forest. Mean crop 

tree growth was not significantly correlated with the presence or absence of shrub 

species, although mean growth was correlated with site factors such as elevation 

and aspect. Five years after herbicide treatment of some of the sites, measurements of crop tree height, leader length, and diameter were not significantly different from those on comparable untreated sites (Horowitz, unpublished data). 

Although these studies provide information on the relative growth of conifers in 

natural situations with and without Ceanothus, they do not address the question of 

how conifer growth might be affected by reduction or removal of Ceanothus. Many 

factors, other than the direct influence of Ceanothus on growth, may influence 

41 
results in nonmanipulative studies such as these and make results difficult to interpret. The open environment may be particularly stressful for young conifers if they 

are growing within the active rooting zone of shrubs, or if water-using herbs are 

present. In addition, shrub regeneration may have occurred preferentially in 

favorable microsites, and confounding factors such as increased or decreased 

animal damage in shrub canopies can influence results. 

Effects of Ceanofhus Much of the information available on this topic has already been discussed in secon Site Quality tion 4 so will not be repeated here. There have been few direct measurements of 

the effect on associated conifers of nitrogen added to the soil by Ceanothus 

species. Youngberg and others (1979), however, report significantly higher levels of 

foliar N in July in Douglas-fir seedlings growing under C. velutinus (1.56 percent N) 

than in seedlings growing in the open (1.15 percent N) or at the edge of Ceanothus 

canopies (1.32 percent N). They consider 1.2 percent N in foliage to be the 

minimum for “adequate growth .” 

A recent study by Binkley and Husted (1983) reports similar results for seedlings 

associated with C. sanguineus. In that study, foliar N levels correlated well with 

levels of available N in the soil. Binkley and Husted also found higher levels of 

calcium and magnesium both under the shrub canopies and in the foliage of 

Douglas-fir growing under the canopies as compared to areas where Ceanothus 

was absent. These studies suggest that, at least on some sites, the presence of 

Ceanothus species can improve the mineral nutrition of associated conifers. Information is lacking on how these differences may affect growth and on what types of 

sites we can expect to show these types of responses. 

There are many areas in the Cascade Range where soil N levels are quite high 

and rainfall input of N is probably on the order of 2 to 3 kilograms per hectare 

(2.7 Ib/acre) per year. Although this adds up to a large amount of N over several 

hundred years, it is probably insufficient to maintain long-term productivity with 

short rotations and frequent slash fires. 

Conclusion Several general observations came out of this session. First, we do not have sufficient data to permit accurate predictions of the effect that management of 

Ceanothus species will have on long-term site productivity, even for sites where C. 

velutinus is dominant. There is considerably less information on other Ceanothus 

species. Second, we have a poor understanding of the tradeoffs involved in controlling Ceanothus. Can reduced conifer growth attributed to competition from 

Ceanothus be balanced by long-term nutritional benefits? Although this is a 

reasonable hypothesis, it has not yet been tested thoroughly. Because of the effects of water stress on N2 fixation, that maximum (or near maximum) levels of 

nutrient addition might be achieved at well below maximum cover of Ceanothus. 

Third, both sampling and experimental studies can add to the understanding of 

Ceanothus-conifer interactions. Whenever possible, information from sampling 

studies should be verified by well-designed experimental studies. Fourth, the 

Ceanothus-conifer systems are extremely complex: Researchers should be aware 

of this and avoid simplistic, single-factor interpretations. 

42 
Research Needs It was generally agreed that many of the past studies on effects of Ceanothus 

species on conifer growth have serious flaws that should be corrected wherever 

possible in future research. Some of the major recommendations of participants in 

this regard were: 

1. Studies should be designed to avoid confounding of factors such as microsite 

differences. 

2. Sufficient time should be allowed for responses to occur before publication of 

results. Because of initial growth check following planting or resulting from release 

treatments, growth responses may not be observed until 3 to 5 years after treatment on many sites. Studies should be designed with at least a 5- to IO-year time 

frame. 

3. Height and diameter measurements should be standard practice. Diameter 

growth frequently responds more rapidly to treatments than height growth. Furthermore, factors such as shading and moisture stress can alter height-to-diameter relati o n s h i ps d ram at i cal I y. 

4. Effects of treatments on shrub growth, distribution, and canopy volume should 

also be evaluated. This information is critical for comparing results of different 

studies, for developing models to predict growth benefits, and to determine where 

and when these benefits can be expected. 

5. Close attention should be paid to the quality and source of planting stock and to 

careful handling and planting of seedlings. 

6. Clear distinction should be made between studies evaluating responses of 

natural regeneration and those involving planted seedlings. 

7. Effects of factors such as animal damage, insect infestation, mechanical 

damage, and frost injury should be recorded and quantified. For example, in a recent manipulative study in C. velutinus brushfields, Perry3 observed substantial 

aphid infestations on Douglas-fir saplings when surrounding shrubs were removed. 

a Personal communication, D. Perry, Forest Science Department, 

Oregon State University, Corvallis, OR 97331. 

43 
7. Research Priorities3 

Six major areas of needed research were identified: (1) autecology and 

physiological ecology of Ceanothus species, (2) interactions between Ceanothus 

and conifers, (3) development of computer simulation models, (4) synthesis of information that can be obtained from stand sampling, (5) interactions between 

Ceanothus and wildlife, and (6) standardization of study methods and nomenclature. Technology transfer is also needed. Research in the first two areas was 

ranked as high priority by all three study groups. The importance of summarizing 

pertinent research information in forms useful to land managers was also discussed, and suggestions were made for implementation. In all research areas, participants stressed the need to obtain more information (1) on species other than C. 

velutinus and (2) from geographic areas outside the Pacific Northwest. There was 

also general concern expressed over the need for well-designed, replicated studies 

that can serve multiple purposes. There is a need for long-term studies that will 

produce information concerning conclusions about the effects of Ceanothus over 

one or more rotations. 

Autecology and physiological ecology of Ceanothus species: 

1. Investigate requirements for and effects of Ceanothus establishment-seed 

longevity, dormancy, germination requirements, effects of fire intensity, seed production and storage, planting of Ceanothus, reproductive strategies (seed vs. 

sprout), and effects of age, size, and site characteristics on sprouting. 

2. Study productivity-determine growth rates, biomass production, site occupancy, 

canopy structure and densities, seasonal growth patterns (phenology), carbon 

balance and allocation patterns, and develop predictive models. 

3. Conduct detailed field research on nitrogen dynamics-nutrient cycling, nitrogen 

fixation, and effects of environmental conditions (water, light, temperature, biotic interactions, micronutrients, and pH) and of endophyte characteristics. 

4. Conduct field investigations of patterns of water use and water relations of 

Ceanothus species, especially for species other than C. velutinus. 

5. Investigate effects of Ceanothus on erosion prevention and slope stability. 

Interactions between Ceanothus and conifers: 

1. Identify existing long-term studies (at least 20 years) with adequate design or 

controls to permit comparisons of sites where Ceanothus is present with those 

where it has been removed. This could provide preliminary estimates of the longterm effects of potential management strategies for Ceanothus-conifer systems. 

3 Participants were divided into three separate groups to identify 

research priorities. This section summarizes the conclusions of 

all three groups. 

44 
2. Establish long-term experiments to look at effects of absolute and relative densities of Ceanothus and conifers on growth, nutrients, and other factors. Both 

replacement series and Nelder designs are approaches that might be useful. 

Weigh positive and negative interactions (nitrogen fixation, browse value, moisture 

competition, and shading). 

3. Initiate well-designed studies to evaluate positive and negative effects of release 

in plantations where Ceanothus is well established (for example, codominant or 

do m i n an t) . 

4. Investigate interactions between conifers and vegetation that replace Ceanothus 

where Ceanothus is controlled. Look at effects of manipulation on seral sequences. 

Weigh benefits against negative interactions. 

5. Investigate effects of timing on interactions; for example, how does the effect of 

a short rotation of Ceanothus followed by its removal and planting of conifers compare with the effect of growing the two species concurrently or eliminating 

Ceanothus entirely? 

6. Monitor nutrients, physiology, and environmental conditions in the studies listed 

above. 

Development of computer simulation models: 

1. Test and integrate scientific concepts or assumptions and generate hypotheses. 

2. Evaluate the importance of individual factors that influence interactions between 

Ceanothus species and conifers, such as competition for site resources, growth 

enhancement, or suppression. 

3. Predict responses of Ceanothus-conifer association to individual factors, such as 

n it ro g e n f i xat io n lacc re t i o n , sit e f e rt i I it y, and m an ag e me n t practices . 

4. Facilitate information exchange and stimulate discussion among scientists, land 

managers, and others. 

Synthesis of information from stand sampling: 

1. Analyze existing stand examination data from a wide geographical area for 

possible use in acquiring site-specific information. 

2. Integrate biological research data with stand examination data for possible use 

in management of the Ceanothus-conifer complex. 

3. Corroborate research results on a broader scale. 

The use of Ceanothus species by wildlife: 

1. Develop guidelines for wildlife management where Ceanothus is abundant. 

2. Conduct additional research on wildlifelCeanothus interactions. 

45 
3. Investigate the effect of shelter, alternate food, and browse for wildlife on conifer 

establishment, survival, and growth. 

Standardization of study methods and definitions: 

1. Define terms and concepts for uniformity; for example, definitions of conifer suppression can incorporate such factors as the percentage of foliage that is above 

the photosynthetic compensation point (more practically, the percentage of foliage 

that is shaded), conifer development over a specific period of time, and height of 

conifer leaders. 

2. Develop standardized, statistically sound study methods and designs that will 

allow comparison of results among experiments. 

3. Develop interim guidelines for Ceanofhus management based on current 

knowledge of the importance of Ceanofhus as browse for game species, as a site 

stabilizer, as a nitrogen fixer, and as a competitor. 

Technology transfer: 

1. Publish a periodic newsletter describing current studies or research results. This 

can include a directory of individuals active in Ceanofhus research and management. Encourage researchers to use existing periodicals (such as Forestry 

Research West, FIR Report, Forestry Update) for disseminating research 

in form at ion . 

2. Develop monograph or book on the biology of Ceanofhus. 

3. Develop a comprehensive manual on the management of Ceanofhus much like 

the silvics manual used by foresters. 

Many of the research needs that are identified here overlap considerably, and with 

careful research design and coordination among researchers in different areas, information on many of these topics can be obtained from single experiments. 

Because of the large investment in time and energy required for large-scale field 

experiments, the consensus was that coordinated interdisciplinary efforts should be 

encouraged wherever possible. 

46 
8. Concluding Remarksw 

Much scientifically and operationally useful information is known about Ceanofhus. 

Everything about all 55 species in all conceivable environments will never be 

known. Thus, we must make an investment decision, or rather two: 

1. What do we do with the information we have? 

2. What new information should we seek, if any? 

The answer to the first question is straightforward. The results of this workshop 

should not only be packaged into a proceedings, but distilled into a “how to” 

publication for people interested in managing or studying Ceanofhus. This publication should be presented as widely as audience demand will allow. Thus, we could 

be reasonably sure what we know about Ceanofhus will be used and not misused. 

The answer to second question is trickier. If we invest in Ceanofhus, we will not invest in some other dollar equivalent research. consideration of what we are willing 

to give up to learn more about Ceanofhus is a potential problem in obtaining financial support for Ceanofhus research. I am willing to hypothesize that until we know 

something about true fir autecology, for example, or how to grow Douglas-fir on 

specific sites or to meet specific objectives, further research on Ceanofhus should 

be defined by two criteria: 

1. What hypotheses, leading to results generalizable beyond the genus Ceanofhus, 

can be tested most efficiently using Ceanofhus as a guinea pig? 

2. What missing practical information about Ceanofhus now prevents us from making effective and efficient management decisions? 

By way of examples, more than concrete suggestions, I put forward two hypotheses 

for addressing the first criterion. 

1. The general hypothesis that endophyte quantity and quality limit nodulation 

might be efficiently tested with Ceanofhus. Nodulation in Ceanofhus is spotty, but 

apparently ubiquitous. Testing will require several activities including isolation of the 

endophyte into pure culture and field experiments where endophyte populations are 

deliberately manipulated. Neither will be easy. 

2. A second general hypothesis is that nitrogen-fixing species employ different 

evolutionary strategies in the occupation of territory than do nonnitrogen-fixing 

species. Comparisons of Ceanofhus with similar but non-nodulating genera could 

shed light on this question. 

3 This section prepared by John Gordon, Dean, School of 

Forestry, Yale University, New Haven, CT 06520. 

47 
To address the second criterion, additional practical information should be sought. 

Ideally, sensitivity analyses should be done to indicate returns from different kinds 

of environmental information such as that done for soils by Schone (1983). 

Because it is unlikely that this will happen, I suggest: 

1. Examine the consequences of control by observing the effects of complete and 

partial control over time on all values of management interest, including wildlife, 

soil erosion, and crop tree growth, in relation to probable future management 

systems. 

2. Learn how nitrogen accretion occurs; the major need is to know relationships 

between the quantity and effectiveness of nodules and aboveground variables such 

as species composition and stem basal area. 

3. Learn how variability at the species level can be used for specific management 

purposes through common garden studies that relate nitrogen fixation to growth 

rates and competitive effects. 

By far, the highest priority for incremental effort is to capture, and to render usable 

by managers, the results of the last 2-1/2 days. 

Acknowledgments 

We thank Dr. Peyton W. Owston and Dr. John Walstad, Pacific Northwest Forest 

and Range Experiment Station, Corvallis, Oregon, for their roles in initiating the 

project and for their continued support and interest. 

We also thank Lois Schuldt and Tami Webb, Pacific Northwest Forest and Range 

Experiment Station, Corvallis, Oregon; and Pam Henderson, Oregon State University, School of Forestry. 

The efforts of the organizing committee, discussion leaders, and those who 

reviewed this manuscript are much appreciated. 

Support was given to this workshop by the National Science Foundation, 

Ecosystems Analysis Section (Grant - DEB-8217509); the Pacific Northwest Forest 

and Range Experiment Station, Forest Service, U.S. Department of Agriculture; and 

the Forest Science Department, Oregon State University. 

48 
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65 
Appendix 1: Scientific Names of Species Cited 

Ceanothus Species Ceanothus americanus L. (New Jersey Tea) 

C. cordulatus Kell. (mountain whitethorn) 

C. crassifolius Torr. (hoaryleaf ceanothus) 

C. cuneatus (Hook.) Nutt. (buckbrush) 

C. diversifolius Kel I. (t rai I i ng ceanot h us) 

C. fendleri Gray (Fendler ceanothus) 

C. greggii Gray (desert ceanothus) 

C. greggii var. perplexans (Trel.) Jeps. 

C. greggii var. vestitus (Greene) McMinn 

C. incanus T. & G. (coast whitethorn) 

C. integerrimus H. & A. (deerbrush) 

C. lemmonii Parry (Lemmon ceanothus) 

C. leucodermis Greene. (chaparral whitethorn) 

C. megacarpus Nutt. (bigpod ceanothus) 

C. oliganthus Nutt. in T. & G. (hairy ceanothus) 

C. parvifolius (Wats.) Trel. (littleleaf ceanothus) 

C. prostratus Bent h. (squaw carpet) 

C. pumilis Greene (dwarf ceanothus) 

C. sanguineus Pursh. (redstem ceanothus) 

C. thyrsiflorus Esch. (blueblossom) 

C. tomentosus Parry 

C. velutinus Dougl. (snowbrush) 

C. velutinus var. velutinus Dougl. ex Hook. (snowbrush) 

C. velutinus var. laevigatus (Hook.) T. & G. (varnishleaf) 

Conifer Species Abies amabilis Dougl. ex Forbes (Pacific silver fir) 

A. concolor (Gord. & Glend.) Lindl. ex Hildebr. (white fir) 

A. grandis (Dougl. ex D. Don) Lindl. (grand fir) 

A. magnifica A. Murr. (California red fir) 

A. procera Rehd. (noble fir) 

Chamaecyparis lawsoniana (A. Murr.) Parl. (Port-Orford-cedar) 

Larix laricina (Du Roi) K. Koch (tamarack) 

Libocedrus decurrens Torr. (incense-cedar) 

Picea sitchensis (Bong.) Carr. (Sitka spruce) 

Pinus attenuata Lemm. (knobcone pine) 

P contorta Dougl. ex Loud. (lodgepole pine) 

P coulteri D. Don (Coulter pine) 

P jeffreyi Grev. & Balf. (Jeffrey pine) 

P lambertiana Dougl. (sugar pine) 

P ponderosa Dougl. ex Laws. (ponderosa pine) 

P radiata D. Don (Monterey pine) 

/? sabiniana Dougl. (Digger pine) 

P taeda L. (loblolly pine) 

Pseudotsuga menziesii (Mirb.) Franco (Douglas-fi r) 

66 
Other Species 

Sequoia sempervirens (D. Don) Endl. (coast redwood) 

Sequoiadendron giganteum (Lindl.) Buch holz (giant sequoia) 

Thuja plicata Donn ex D. Don (western redcedar) 

Tsuga heterophylla (Raf.) Sarg. (western hemlock) 

Trees and shrubs: 

Alnus Mill. (alder) 

Arctostaphylos pa tula G re en e (g re e n I e af man za n it a) 

Castanopsis sempervirens (Kell.) Dudl. (bush chinkapin) 

Holodiscus Maxim. (ocean-spray) 

Lithocarpus densiflorus (Hook. & Arn.) Rehd. (tanoak) 

Prosopis glandulosa Torr. var torreyana (L. Benson) M.C. 

Prunus L. (cherry, plum) 

Quercus L. (oak) 

Quercus vaccinifolia Kellogg (huckleberry oak) 

Quercus kelloggii Newb. (California black oak) 

Jo h nst (m esq u i te) 

Mycorrhizal and symbiotic fungi: 

Armellaria mellea Vahl. ex Fr. (Armellaria root disease) 

Cenococcum geophilum Fr. (Cenococcum) 

Eutypa armeniacae Hansf. & Carter. (no common name) 

Frankia spp. (Frankia) 

Laccaria laccata (Scop. ex Fr.) Berk. & Br. (Laccaria) 

Rhizobium japonicum 

67 
Append i x 2 : Workshop Part i c i pant 

Ray Ar mst ro n g , Si lvic u It u r i st 

Umpqua National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 1008 

Roseburg, OR 97470 

Tom Atzet, Area Ecologist 

Siskiyou National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 440 

Grants Pass, OR 97526 

Tom Beebe, Silviculturist 

Wenatchee National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 811 

Wenatchee, WA 98801 

Dan Bin kley, Assistant Professor 

School of Forestry and Environmental Studies 

Duke University 

Durham, NC 27706 

Raymond J. Boyd, Silviculturist 

Intermountain Forest and Range Experiment Station 

USDA Forest Service 

1221 South Main 

Moscow, ID 83843 

Jim Boyle, Professor 

School of Forest Management 

Oregon State University 

Corvallis, OR 97331 

Susan Conard, Ecologist 

Pacific Southwest Forest and Range Experiment Station 

USDA Forest Service 

4955 Canyon Crest Drive 

Riverside, CA 92507 

Kermit Cromack, Associate Professor 

School of Forest Science 

Oregon State University 

Corvallis, OR 97331 

1’ Current addresses are provided for all participants. 

68 
Valerie A. Davis, Biologist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Edward J. Dimock, II, Silviculturist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Paul Dunn, Project Leader 

Pacific Southwest Forest and Range Experiment Station 

USDA Forest Service 

4955 Canyon Crest Drive 

Riverside, CA 92507 

Harold Evans, Professor 

Department of Botany 

Oregon State University 

Corvallis, OR 97331 

Jerry Franklin, Project Leader 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

R.L. Fredrickson, Soil Scientist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

John Gordon, Dean 

School of Forestry and Environmental Studies 

Yale University 

New Haven, CT 

Charles Halpern, Research Assistant 

School of Forestry 

Oregon State University 

Corvallis, OR 97331 

Ed Harshman, Forester 

Willamette National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 10607 

Eugene, OR 97526 

69 
Miles Hemstrom, Area Ecologist 

Willamette National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 440 

Eugene, OR 97526 

lvend Holen, Silviculturist 

Butte Falls Resource Area 

Bureau of Land Management 

3040 Biddle Road 

Medford, OR 97501 

Howard Horowitz, Assistant Professor 

School of Environmental Studies 

Ramapo College of New Jersey 

505 Ramapo Valley Road 

Mahwak, NJ 07430 

An nabel le Jaram i I lo, Botanist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Martin Jurgensen, Associate Professor 

Department of Forest Soils 

Michigan Technological College 

Houghton, MI 49931 

B r u c e Ke I ps as, Reforest at i o n Cons u It ant 

Sylvan Reforestation Consultants 

2750 NW Royal Oaks Dr. 

Corvallis, OR 97330 

Paul Lauterbach, Consulting Forester 

Lauterbac h Forestry Services 

1140 N. Cascade 

Tacoma, WA 98406 

Denis Lavender, Dean 

Forest Science Department 

University of Butesh Columbia 

Vancouver, BC, Canada V6TlW5 

W i I I i am Lopus h i n s ky, PI ant Physiologist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

1133 N. Western Ave. 

Wenatchee, WA 98801 

Hank Margolis, Graduate Student 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

70 
John Marshall, Research Assistant 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

Phil McDonald, Silviculturist 

Pacific Southwest Forest and Range Experiment Station 

USDA Forest Service 

2400 Washington Ave. 

Redding, CA 96001 

Dave McNabb, Forest Engineering Specialist 

Forestry Intensified Research (FIR) 

1301 Maple Grove Dr. 

Medford, OR 97501 

Don Minore, Plant Ecologist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Logan Norris, Head 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

Peyton W. Owston, Project Leader 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Terry Petersen, Research Forester 

Cham pion International 

P.O. Box 8 

Milltown, MT 59851 

Greg Prull, Graduate Student 

Geography Department 

University of Oregon 

Eugene, OR 97403 

Steve Radosevich, Associate Professer 

CRAFTS 

Oregon State University 

Corvallis, OR 97331 

Doug Robin 

Umpqua National Forest, Pacific Northwest Region 

USDA Forest Service 

Glide Ranger District 

Glide, OR 97443 

71 
Sharon Rose, Soil Scientist 

517 Brown Avenue 

Fort Collins, CO 80525 

Bob Sanders, Silviculturist 

Willamette National Forest, Pacific Northwest Region 

USDA Forest Service 

Box 10607 

Eugene, OR 97440 

W i I I iam Sc h lesi nger, Associate Professor 

Department of Botany 

Duke University 

Durham, NC 27706 

Phil Sollins, Research Associate Professor 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

William I. Stein, Plant Ecologist 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

John Tappeiner, Associate Professor 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

Robert 0. Tinnin, Professor 

De part m e n t of B io I og y 

Portland State University 

Box 751 

Portland, OR 97207 

Bob Wagner, Research Assistant 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

John Walstad, Associate Professor 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

Howard Weat herly, Forestry Technician 

Pacific Northwest Forest and Range Experiment Station 

USDA Forest Service 

3200 Jefferson Way 

Corvallis, OR 97331 

Diane White, Research Assistant 

Department of Forest Science 

Oregon State University 

Corvallis, OR 97331 

72 *GPO 596- 738 1985 
Conard, Susan G.; Jaramillo, Annabelle E.; Cromack, Kermit, Jr.; Rose, 

Sharon, comps. The role of the genus Ceanothus in western forest 

ecosystems. Gen. Tech. Rep. PNW-182. Portland, OR: U.S. Department of 

Agriculture, Forest Service, Pacific Northwest Forest and Range Experiment 

Station; 1985. 72 p. 

This report was developed from discussions on the role of Ceanorhus in 

western forests that took place at a workshop held November 22-24, 1982, at 

Oregon State University, Corvallis, Oregon. The workshop provided a forum 

for discussing research relevant to Ceanothus management. Major topics 

were autecology and synecology; interactions between Ceanothus species 

and the soil system; the physiological ecology of Ceanothus and associated 

conifers; and the effects of Ceanothus on forest productivity and growth. The 

workshop was attended by scientists and land managers from government 

agencies, universities, and forest industry. This report summarizes available 

information concerning the biology and ecology of Ceanothus species in 

western forest ecosystems; describes and critiques methods, results, and 

conclusions of past research; discusses current investigations; identifies 

research needs; and includes an extensive bibliography and a listing of 

workshop participants involved or interested in Ceanothus research and 

manage men t . 

Keywords: Ceanothus, competition (plant), ecology (plant), autecology (plant), 

synecology (plant). . 
The Forest Service of the U.S. Department of 

Agriculture is dedicated to the principle of multiple 

use management of the Nation’s forest resources 

for sustained yields of wood, water, forage, wildlife, 

and recreation. Through forestry research, 

cooperation with the States and private forest 

owners, and management of the National Forests 

and National Grasslands, it strives - as directed by 

Congress - to provide increasingly greater service 

to a growing Nation. 

The U.S. Department of Agriculture is an Equal 

Opportunity Employer. Applicants for all Department 

programs will be given equal consideration without 

regard to age, race, color, sex, religion, or national 

origin. 

Pacific Northwest Forest and Range 

Experiment Station 

319 S.W. Pine St. 

PO. Box 3890 

Portland, Oregon 97208